I think the idea that AM and 27 Mhz is 'bad' is going a bit too far. In most cases, an AM 27 Mhz system will work just fine.
The thing about 2.4 Ghz and water is that it doesn't work very well THROUGH it, but works just dandy OVER it. The sub guys will have -very- little use for 2.4 Ghz, but the surface guys should do just fine with it. That has to do with the 'penetrating' ability of RF and how it's related to frequency (very low frequencies work through water better than very high frequencies).
- 'Doc