I have seen this on several boats. You are going to have to pull the boat and let it dry out to get anything to really work long term. The 5200 is great stuff and will work fantastic on your thru-hull fitting but, again, everything has to be thoroughly dry. Remove the thru-hull, dry out, replace thru-hull using 5200, and let cure before launching. Just remember, 5200 is a pain to remove once it is there. You could use 4200 which would be easier to work with later if you needed.
About your crack, again, pull the boat. From the outside, grind the area out and allow to dry. Make sure you grind it out well past the crack and at a good enough angle to allow new material to have good adhesion. After the area dries, could take a several days, you can make a repair. I like West System. I fixed one on a friends boat a few years ago and it went like this. After all dry, sand the area, mix epoxy(use fast hardener), wet area out with small brush, inject epoxy into crack using syringe, mix filler with left over epoxy to a consistency of peanut butter, fill crack with this below the repair finish level, allow to cure overnight. Next, sand area smooth and prepare for adhesion, using glass tape or alike, mix epoxy, wet out area again, lay three layer of glass, each layer being a little bigger than the last. Make sure glass is wet thoroughly each time. Allow to dry. Sand smooth and fair as required. That crack should never come back.
West System has a little kit that comes with the epoxy and hardener, all premeasured in little packages, mixing cups, fillers, sticks, brushes, etc. It comes with six packages of epoxy and runs about $25.00. The glass tape runs about $10.00.
www.westsystem.com
John