You mean the robot will drive on the bottom of the pool of water right?
And if you search the forum for waterproofing a servo you will find a topic admin put up.
If your servo get hot its no point in running it uintil it stops, if it gets hot stop at once, trust me i know all about it, i ran a RC microcontroller at to much voltage and it got HOT and started to smoke.
Yeh u could try 9 V or just buy a better servo..!
(Check out unitedhobbies.com, i have ordered some servos from there and they have got good respons)