Hitec has a line of Waterproof servos, a standard strength servo should be fine
A Y harness is the common method, but I have seen a few guys with these larger birds actually use a separate RX and battery to isolate the water rudder servo from the Flight RX..
I use a the RcXel kill switches in all my gas planes, Easy to set up, cheap and so far for me very reliable
http://www.rcuniverse.com/market/item.cfm?itemId=880656
Good luck