All 300 series stainless is very risky to use underwater. Even 316 used for drive shafts can have serious crevice corrosion issues where water
is present but there is a lack of oxygen. This occurs in the area of the cutless bearing and inside the stuffing box. This is more common on boats that sit and are not used often. The only good choice for shafts is Aquamet 22 which has a much higher chromium content.
Welding stainless causes another issue - weld decay - which is caused when the wrong SS alloy is welded. Any stainless to be welded should be designated as 316L with the L meaning low carbon. If not you basically end up with steel along the welds, not stainless.
The best information I have seen recently about this is the article by Steve D'Antonio in the Dec/Jan 2014 issue of Professional Boatbuilder
Stainless through hulls are used on steel boats as bronze here is an issue.
Marelon is ABYC approved but wouldn't be my choice. It is nylon based Dupont Zytel. The handles are weak, especially on the smaller sizes. Another issue is the size hole required as the marelon through hulls need a larger hole through the hull
for a given through hull
size. This adds much complication if you wish to return to bronze in the future, involving re-glassing and then drilling the correct size hole for the bronze through hull. This thread, post 23 is informative: Seacocks... MARELON or BRONZE