A little thread necromancy, but sub clubs use lower frequency radios. the 27 mhz radios should work the best for subs as they are in the HF band. As the frequency increases, the ability to penetrade water decreases; this is why real military subs use VLF-ELF for communications. The antennas have to be massive, but the RF will penetrate to a greater depth.
Our old radios not counting the 27 mhz HF radios (50, 72 and 75 mhz) are all VHF. They penetrate water to a certain extent, but not to a great depth. As you probably don't want your subs going too deep, these are a good option and probably what most sub guys use because they aren't hard to come by.
2.4ghz is S-band microwave. That's why they have such short antennas; they don't need a huge antenna because the wavelength is significantly shorter. The signal doesn't penetrate water though. The recon aircraft I flew in the Army carried an X-band RADAR which we sometimes used for RADAR imagery. Water came back as a huge black hole. Same deal with the Ku-band radar that we used from time to time. Microwave energy just skips off the water and dissipates.
It's been a long time since my radio and radar theory classes, so I might have some of the exact details wrong, but when dealing with water penetration, the essentially bottom line is, the lower frequency, the better it will work.
EDIT: To be more on topic, there are some 2.4ghz modules you can get to hack onto your old 72 mhz radio and make them work at 2.4 ghz. I haven't tried one yet, so I can't speak for whether they are any good or not. Here are some links:
http://www.hobbyking.com/hobbyking/store/__9770__Corona_2_4Ghz_DIY_Module_RX_DSSS_.html
http://www.hobbyking.com/hobbyking/store/__8354__X8D_2_4GHz_trainer_port_X8_hack_module_PPM_Only_.html
Here's a link to a thread where someone details adding the Assan module to his old futaba skysport 6:
http://www.rcgroups.com/forums/showthread.php?t=970828