The best widely available image intensifier is the PVS-14. You also would want the 3x lens that is made for it. ITT was the initial manufacturer for the US and Nato military. It is export controlled at the level that you can keep it on your boat while travelling overseas but you can't sell it or lose it overseas, have to take responsibility for it, and you have to bring it back to the US. These are Gen III and expensive, but if you can afford it they are the right tool for the job.
The IR units like those made by FLIR work great if you're looking for people in the water or icebergs, o