Sounds like a great project.
Some additional comments:
1. your 3G signal will not likely penetrate the surface of the water -- at least not more than a foot or two. So your "3G Webcam" will somehow need to extend its antenna to the surface.
2. "green" methods of electricity are very expensive, and likely cost prohibitive. It's a nice idea that you could use solar or wind, but the challenge becomes storing that energy for when the wind ain't blowin' and the sun ain't shinin'. Unless of course, you only want your images being uploaded when your method of power is happening.
Once you get to the stage of storing this power in batteries, you end up with some crazy-expensive power converters and regulators, blah, blah, blah... there just isn't room to do it without some box above water.
Your best bet would be to do the same thing as your Bonaire friends have -- use PoE and an Ethernet connection for your webcam, to the nearest building capable of handling that.
As for the "not bolted down" comments -- when people risk their lives to break into BC Hydro substations to steal mass amounts of copper, I think something like this (even the aforementioned antenna) is at grave risk of being damaged or removed. Never underestimate the power of stupidity!
