Lower frequency signals (who would have thought a few years ago that 75MHz was "lower") can cope with underwater situations as long as it is fresh water. 2G4, as long as the end inch of the receiver aerial is above water, is fine for all practical purposes. Most of the Laser racers around here use the HK radio (cheap and effective), usually with the aerial taped under the centre of the plastic deck without problems. Having it below the water surface level does give problems. The long aerial wire of older systems was difficult to mask - there was always some of it "visible". The tiny 1" bit of 2G4 aerial is much easier to accidentally hide behind, say, a metal mast.
I avoid anything that says "park flyer" because that translates to short range. I just don't like the idea of "antenna free" because that implies that the antenna in inside the box, making good siting difficult. Any radio that has 4-6" of wire poking out of its box should give very adequate siting choices.