I had problems judging direction at 400 yards with a 1 metre sailboat carrying a B rig. Just seeing something not carrying a big marker like that at range is a problem especially is the water is a bit lumpy.
The heli lads use GPS and a magic box to return their toy to its start point, but it doesn't take account of any obstructions that might be in the way, which is much more likely in water. I don't know if manual control is restored if radio communication is restored.
Before FPV gets a mention, it has its own limitations, range being the first. The other is that, according to a guy who was trying one recently, you have little or no depth/distance perception through the viewer. This from the UK RC Laser champion just after he rammed the bank. The low viewpoint didn't help a lot, either.