Joe is correct! My inexpensive mono scope is waterproof and floats. It is even yellow unlike many waterproof gadgets. It cost me under $150.00 and that may not sound like inexpensive but if you have ever had to dive to untangle a lobster pot warp at night, in windy conditions, alone it is a bargain! I use it to scan ahead and if I see something in the ambient light it has an IR beam that, with the press of a button, lights up whatever you are looking at. The range is probably 100 yards so traveling at sailboat speed is OK for this little gem.
If you navigate at night through lobster pots or just in a channel that requires some care a night vision scope will become one of your treasures aboard.