Water in a starter isn't good. Fresh water is certainly preferable to salt water.
With salt water, corrosion is the issue. First on the commutator, which can usually be cleaned up. Second is the brush springs, which when they corrode no longer keep the brushes in contact with the commutator.
Open it up, dry it out, then see what you've got. You might get lucky - dodging a bullet is a good thing!
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