You could have a bad field coil in the starter.

Seeing you didn't ssay, I'm guessing this is a late 70's to early 90's
mercruiser with the GM engine.

Well, GM/Delco in all their great wisdom decided that when copper was a
buck a pound, and aluminum was .40c a pound. they decided to make the
fields out of aluminum. which eventually corrodes and doesn't allow
current to pass though it effectivly, especially when hot. If yiou take
the starter apart, which is no great task, you might find that the
fields are aluminum, and you can replace them yourself with regular
automotive copper fields. it's really not that hard of a task. either
that, of find a field barrel that has the copper fields already
installed. again, not a difficult task.

That will give you an excellent oppertunity to inspect the brush's as
well.

BTW, there is no diffrence between the automotive and the marine field
set. the only thing on the field frame, is the grommet on the solonoid
lead wich is a better seal than the aautomotive type grommet.

Tim