Well you have a few things you can do.
1) Use a CB system, esp. SSB, if you are gonna be out there on 'remote' islands, the 27 MHz band should be pretty free, and you might just be able to use a few junk radioshack CB radios (just make sure you get a nice antenna) and make it work.
2) Use a MURS system, the new MURS channels are pretty much used all over the world now (illegal yes, but FRS is illegal in many places, and it is used)
The MURS radios are pretty much VHF business band frequencies that are license-free. My advice would be to get some nice VHF base stations, and some nice virtical antennas (Ringos, maybe?) and then use the MURS channels (151.820, 151.880, 151.940, 154.570, and 154.600 MHz). With a nice antenna, 20 miles should be no problem.
3) Use a GMRS system, although 20 miles would be a strech, because GMRS is UHF, i would go with either the CB system or MURS system. (the CB idea would be cheaper.
As far as Marine HF goes, it is overkill, unless you use it on the CB band (26.965 to 27.405) or the Business Radio HF Channels (27.430, 27.450, 27.470, 27.490, 29.710, 29.730, 29.750 MHz)
thanks, and if you have any questions, e-mail me atCAPcadet69@hotmail.com