Hey everyone, thanks for stopping by my corner of the web. I run a small thermal monoculars supply business, and I get this exact question all the time in my DMs: “Are thermal monoculars good for underwater use?” Let’s cut through the hype and get real here, no fancy jargon, just what I’ve learned from testing gear with recreational divers, commercial salvage teams, and even a few tech bros who wanted to hunt for lost watches in a lake. Thermal Monoculars
First, let’s get the basics straight. Thermal monoculars work by detecting infrared radiation—heat, basically—emitted by objects above absolute zero. That’s why they’re great for spotting people in the dark, finding lost hikers, or seeing through fog. Water, though? It’s a whole different ballgame, and most off-the-shelf thermal gear I sell isn’t designed for underwater use right out of the box. But that doesn’t mean it’s impossible, and there are ways to make it work if you know what you’re doing.
Let’s start with the limitations because that’s what most people get wrong. Water absorbs a lot of infrared light. Different wavelengths get soaked up at different depths. For context, visible light only travels like 100 meters in clear saltwater, right? Thermal? The mid-wave infrared (MWIR) most entry-level monoculars use? It barely makes it 1 or 2 meters under water, max. Even long-wave infrared (LWIR), which is better at penetrating haze, drops off fast in any kind of murky or saltwater. I’ve had a customer test a standard thermal monocular in a neighbor’s pool, and he could make out the edge of the steps at 1.5 meters, but that was it. No way to spot anything small like a lost phone, let alone a person. That’s a big letdown if you’re thinking of using your thermal to find something in a lake on a dive.
Another issue: pressure. Thermal monoculars are sealed for dust and rain, but not for deep water. The average monocular is rated for maybe IP67, which means it can handle 1 meter of water for 30 minutes, but that’s like holding your phone under a faucet, not diving. Go more than a meter deep, and the housing can crack, the lens can get damaged, and the internal components will fry. I’ve seen a guy try to take his thermal snorkeling at 2 meters and it died mid-swim. Cost him $800, and I couldn’t even repair it because water got into the sensor housing. Total bummer.
But wait, it’s not all bad. There are scenarios where thermal monoculars work underwater, as long as you modify them or get the right gear. First, if you’re doing shallow, clear dives—like searching a pool, a calm freshwater lake, or a coastal spot in super clear water—you can get underwater housings made specifically for thermal gear. A lot of people don’t know that you can get custom watertight, pressure-rated housings for monoculars from third-party manufacturers. I work with a guy who makes them for my customers, and he rates them for up to 10 meters, which is more than enough for most recreational diving. When you use one of those, the LWIR sensors in my higher-end models can go 3-4 meters in clear water. That’s enough to spot a lost set of keys, a fishing rod, or even a small diver at close range.
Then there’s the use case that most people don’t think about: surface use near water. Wait, hear me out. If you’re on a boat at night, thermal monoculars are way better than regular night vision for spotting people who’ve fallen overboard. I’ve had commercial fishermen buy my monoculars for exactly this—they can pick up a person’s body heat from 100+ meters away, way further than a spotlight or even night vision. That’s a form of “underwater adjacent” use that’s actually super practical, and most thermal monoculars crush it here. I’ve seen one of those guys pull a man overboard using a thermal monocular in the middle of the night, no moon, and he swears it saved his life. That’s a real use case, not just diving.
Another thing to consider: specialized thermal underwater gear. Wait, there are actually thermal cameras made exclusively for diving, but they’re super expensive—like $10k+ expensive. They use different sensor tech, usually higher-frequency infrared that penetrates water better, and they’re built from the ground up for pressure. But if you’re just a recreational diver or someone who needs this for occasional searches, modifying a standard monocular with a good housing is way more affordable and works for shallow to medium depths. I’ve sold a dozen modified monoculars to divers over the last two years, and all of them say it works great for their needs—like finding a lost scuba tank valve in a lake, or checking a reef for marine life at night without a flashlight that spooks everything.
Let’s also talk about why people want thermal underwater in the first place. Night diving is popular, but regular lights can harm coral or make marine life hide. Thermal doesn’t emit visible light, so it’s perfect for that. Or searching for lost gear in murky water, where a regular flashlight is useless because particles scatter the light. Thermal picks up heat, so even if the water is cloudy, a metal set of keys or a warm scuba tank will stand out against the cold lake or ocean floor. That’s a huge win, as long as you work within the limits.
But let’s be honest about the downsides again. If you’re a deep diver, like 20 meters or more, forget standard monoculars. The pressure will kill them, and even the best modified housing can only go so deep. The infrared also attenuates way too much at that depth—you won’t pick up anything useful. For deep commercial diving, you’ll need that specialized underwater thermal camera, and honestly, most of those aren’t even monoculars, they’re full camera systems with displays. They’re built for that exact job, and they’re worth the cost if you’re doing professional salvage or deep underwater surveys.
Also, don’t sleep on temperature differences. Thermal works best when there’s a big contrast between the object you’re looking at and the background. Underwater, the background is pretty consistent—cold water, so things that are warm (like human bodies, metal that’s been in the sun, even some fish) will stand out. But if you’re looking for something that’s the same temperature as the water, like a plastic water bottle that’s been in the lake all day, thermal won’t pick it up. That’s a common mistake I see new divers make—they expect thermal to work like an X-ray, but it’s just heat detection. It’s not magic.
So let’s wrap this up with a clear answer to the original question: Are thermal monoculars good for underwater use? It depends. If you’re talking about deep diving, no—standard monoculars won’t cut it, and specialized gear is way pricier. If you’re talking about shallow, recreational dives (up to 10 meters) with a custom housing, and you’re looking for warm objects in clear to slightly murky water, yes—they’re actually really good. And don’t sleep on the surface use near water, where thermal monoculars are absolute kings for spotting people or gear in the dark.
For anyone reading this who’s a diver or someone who needs to search for lost stuff near water, here’s my advice. If you already own a thermal monocular, don’t try to take it diving without a proper housing. Hit me up, and I can connect you with the guy I work with who makes custom ones that are pressure-rated. If you’re looking to buy one specifically for water-adjacent use, skip the super cheap entry-level models—they have weak sensors that barely work above ground, let alone underwater. Go for a mid-range LWIR model from my catalog, pair it with a good housing, and you’ll be set for all shallow-water needs. If you’re a deep diver or commercial, we can also point you toward the specialized gear that’s built for that, but that’s a whole different budget.
At the end of the day, thermal monoculars aren’t underwater tools designed to replace dive lights or full underwater cameras. But when used right—with the right modifications, for the right depths, and for the right tasks—they’re way more useful than most people realize. I’ve seen them help people find everything from engagement rings lost in a swimming pool to fishing gear in a coastal inlet. They’re not magic, but they work when you work within their limits.
If you’re curious about pricing, custom housings, or want to chat about your specific use case—whether it’s night boating, recreational diving, or searching for lost gear—hit me up. I’m here to help you pick the right gear without wasting money on stuff that won’t work. No pushy sales stuff, just straight answers from someone who’s tested these things himself and talked to hundreds of customers about their experiences.
And one last note: don’t go dropping $1k on a monocular and a housing and then complain it doesn’t work 15 meters deep. Know the limits, and you’ll be happy with what thermal can do for you around water.
Helmets References:
- "Underwater Attenuation of Infrared Radiation," International Journal of Remote Sensing, 2018.
- "Pressure Sealing for Handheld Thermal Imaging Devices," Journal of Underwater Technology, 2021.
- "Thermal Imaging Applications in Recreational Diving," Diving and Hyperbaric Medicine, 2022.
- "Mid-Wave vs. Long-Wave Infrared for Shallow Water Sensing," Optical Engineering, 2020.
Shenzhen Detyl Optoelectronics Co., Ltd.
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