Do not coil the antenna! I've seen a number of planes go down because a novice hadn't uncoiled the antenna when he put the RTF plane together, went to the field and put it in hard!

Dragging the antenna in the water won't hurt it, though I don't know what kind of set-up you have that wouldn't allow some means of keeping it out of the water. If you are thinking of coiling the antenna and then attaching it to the vertical stab, why not just attach it to the vertical stab without coiling it? Wouldn't this keep it up out of the water?