To answer the specific question, I'd use 3M 4000UV (vs 4200) for an above-the-waterline thru-hull.
It has just become my go to for most things above the water line, because I find it more universal, e.g. works better on metals than 4200, and doesn't yellow in the sun like 4200. I don't even keep 4200 on hand anymore.
Having said that, I'll admit that I use DynaFlex 230 to seal acrylic into ports, around vinyl windows, and similar. I know it isn't marine rated, so I don't recommend it, but I somehow got started using it, and it hasn't let me down. It seems to stick to everything and be super flexible, so when things want to flex, it still gets it done when other things let go or dry out and brittle in the sun and then let go. I've never had it fail. But, I do use it only in those limited cases, and wouldn't use it on a thru-hull.
As for 5200, I use it only below the water line. It doesn't stick to some plastics, in particular some vinyl windows I've tried to use it on, and some metals, in particular some aluminum windows I tried to use it on. Where it does bond, it doesn't flex, which has sometimes caused flexing substrate to fail, e.g. a top layer come off, and with it the seal, where more flexible sealants have succeeded. It is thinner than other sealants and gets all over, especially when my sloppy self uses it. And, even the fast cure is a slow cure. I've seen the center of thick beads uncured days later, maybe even weeks. There is a lot made of the fact that, when it does bond, it is permanent, but personally, I put that in a plus. I'm glad it doesn't let go of the stuff underneath. And, when I've had to remove it, I've just cut and pried and cleaned up. Hasn't always been fun. But, has always gotten done.
