1. The Dekas Head is not always under water while rolling, only for a very short period of time cause of their rolling speed, so its like under,over,under,over...
2. "If it rains hard they could use their personal shield" and what do the other droids do, like the Pilot Droid or the SBD? do they have an integrated "Arm-Umbrella" for rainy days?
3. Of course u can walk with the deka through the water but if the island is ur only Spawnpoint left (on Kashyyk Docks) Droideka players are doomed to play anything else cause if they would walk the whole way they will reach the beach when the battle is over already