Don't take my word for it, but at 2 ghz with a high quality video and audio
transmitter/receiver (1 watt +/-) with 40 feet of air between them you can
block the signal with a sheet of paper. You need an unobstructed line of
sight path. With water, who knows, but I think lower frequencies would be
easier. There are a lot of low cost video transmitters in the 70 - 300 mhz
range. Check the security and surveillance web pages.

Rogerio