My Bimini is of the urethane type and it shure is watertight. I have never treated it with any repellent stuff but it still shows some of the normal mold on the uderside which I have to tackle next.
All my canvas is in a great shape and only the front window seems to need a repellent treatment after the winter (it faces north north west so it does not get that much sun and the rest is pretty much under cover. The beauty of covered slips here. Of course it is not in a barn wrapped in vinyl like so many other boats up north are.
today was a great boating day with a few gusts at 20+mph but otherwise perfect with 70+ air temp and water around 52 (normal year round for the northwesterners!!