I certainly realize how late this reply is however I thought I might chime in...
Recently my bilge was taking on water big time. I live on the Parker Strip in Arizona and we like to put our boats in the water and leave 'em over night on occasion. All summer my bilge would fill up a few gallons of water within a few hours. I had an automatic pump in there luckily so no problem but I could not find source of leak. Meanwhile, my U- bellows would fill up with water and thus started to rust the gimble bearing. So that being said, I knew I had to pull drive and replace bearing while thinking bellows needed to be replaced in case it was the source of leakage...
Anyway, while taking drive off, a three inch chunk of the round rubber O-ring that resides around and inside the U-joint assembly came flying out. This piece of rubber O-ring had gotten in the way of securing a truly secured mating surface last time I had installed the outdrive unbeknownst to me at the time. Anyway, upon reinstallation of new gimble bearing and out drive, I water tested the boat and gimble was smooth and quiet. Upon pulling her out of the river, I pulled the plug and low and behold...not one drip of water came out!
So this could possibly be a source to check when taking on water in the bilge...never would have noticed otherwise. Just sayin'... but what do I know?