They use cathodic protection which makes the underwater hull basically immune to corrosion. But this doesn't stop water that gets on top or inside.

Ships rust like hell. I've known some guys in the Navy and they said it was common for everything unarmored to have holes all over.

What's their maintenance like?

Scrub/repaint every few years?

In comparison, splashdown capsules spend maybe few hours tops in the water.

The problem is that, unlike ships, they're generally not designed to be water proof. It's not the exterior that's the problem.

I mean, the crew part of it is water proof, right? It has to be air tight...

I'm guessing there are things on the "outside" that might still have issues?

The first part of your post reminded me of this quote from Futurama.

Professor Farnsworth: Good Lord! That's over 5000 atmospheres of pressure!

Fry: How many atmospheres can the ship withstand?

Professor Farnsworth: Well, it was built for space travel, so anywhere between zero and one.

> ... there are things on the "outside" that might still have issues ...

... on the second reentry attempt.