They actually filmed the sequence in TPM where Qui-Gon berates Obi-Wan for shorting out his sabre by leaving it on when immersed in water- that is why Obi-Wan is running from the STAPs in the Naboo forest after he and QGJ get down there, the ones that Qui-Gon destroys just after meeting Jar-Jar.
It's also in the screenplay and the novel. It's not utterly canon but it is a VERY good lead. It's posible GL has changed his mind but I would prefer firm confirmation of lightsabres being used underwater after all that.
I rather think my lead is a better one as it was in a script written by GL (thereby having some canon status) and filmed, which gives more of a clue to his mindset than a book written by Alan Dean Foster, so removed from any form of continuity that not even the EU considers it part of its storyline.