As far as cheap goes, it would be an AM radio system but spend a few more dollars and go FM. There's another thread going on in here discussing a low-dollar 2.4 Gig system but how it will work on water, I don't know. The 27 Mhz band is more prone to getting hits if you run where there might be CB radio traffic such as near highways.
I have no problem running FM as most racers are going to the 2.4 Gig band so that opens it up more and I'm still not 100% sold on them yet.