I havent done it myself but it should be real easy to set up,
as said previously on this thread use a microphone preamp such as this one from Maplin
and a waterproof microphone
or if you are feeling adventurous and the route i would probably try you could use an old VHF microphone ( ebay? ) and re wire it with the switch in series with the microphone to activiate the Mic when you want to speak.
Or if you are really flash this unit looks interesting