how quickly they forget. lightsabers worked underwater in JK, also in Splinter of the Mind's Eye. So, there are 2 examples of lightsabers being used under water. in The Phantom Flop they never needed to use their sabers underwater.
Speaking of the Phantom Flop, why did the gungans build a city underwater???? if they can breath underwater, why bother building their underwater city inside of giant air domes?? and another stupid thing from TMP... nahhh, why bother, I could go on all day.