There is a small amount of rust on the caps of my u-joints. The rust is only on the end caps and it appears to be just surface rust. Is this something I should just replace or are they fine to just grease and go. The boat has 270 hours and Im not sure if these are the originals or not. Thanks.

No big deal as long as the joints are OK. When you grease them watch and see if any water comes out. If so think about replacing them.

How did they get wet?

I'd also look into how the water got in there as US1 mentioned. Being you already have the drive off I'd go ahead and replace it. Cheap insurance. It may be ok for now but surface rust will continue to eat through over time.

Verify that there is no water in the creases of the bellows which would indicate water getting in. Otherwise grease and go like mbam said.

is it possible they are getting hot and condensation when they cool is causing the rust?

Condensation forms on cold surfaces. So, not only would the hot caps have to cool to ambient, they would have to heat removed beyond ambient.

i am thinking hot parts cold water condensation?

There was no water in the bellows and the is no rust on the gimbal bearing either. There is also no rust on the main body of the u joints just the caps which I do find odd myself. I do boat in saltwater, could it just be the air? I know a car will rust out over time. I will grease and see if water comes out thanks for the help.

Condensation only forms on parts colder than dewpoint.
Never seen a coffee cup with condensation, only cold drinks.