Use a passive heat pipe that runs from the sensor to a cold spot on the hull – the water itself is the coolant, no power needed. Make the pipe copper or aluminium for high conductivity, and keep the fins thin so the seawater can flow over them naturally. If you can, embed a phase‑change material in the sensor housing that melts when the sensor heats up and solidifies when the water cools, absorbing heat without any circulation. Finally, seal the sensor enclosure well; a tight seal lets the surrounding water do all the convection for you.

Great point on the pressure test – at 10 m depth you’re looking at roughly 1 bar extra, so use a flange with a 2.5 mm gage ring for 1.5 bar rating. The copper pipe I use is 12 mm OD, 1 mm wall, gives about 0.4 °C per watt; that leaves a good safety factor if your sensor peaks at 2 W. For the fins, keep the spacing to 3 mm so the water still passes easily; that keeps the fin thermal resistance below 0.2 °C/W. The phase‑change material, 60 °C melting point, absorbs about 200 J/g, so a 5 g slab will hold the heat for a couple of minutes. Keep all joints clean and colour‑code the cable runs so you know exactly which pipe is which when you do the pressure test.

Glad the numbers work for you. Run the simulation at the 10 m pressure point and keep an eye on the thermal resistance – a slight bump can throw off the heat‑pipe efficiency. Once you confirm the 5 g slab holds the peak, you’re all set. Good luck with the test, and remember: a clean seal is as crucial as the copper pipe.