The only thing I can say is if water is getting into it, you need to put some type of shield over/around/under or whatever to prevent that. If you take the starter off (2 bolts & an electrical connector), you should be able to determine if water is getting into it. A frozen starter - maybe if there was water in it; but if so, how did the water get in. A starter just doesn't freeze and not work. There is nothing I know of to freeze. My better guess is, that the starter solenoid is shot. But without knowing/hearing what it sounded like when it wouldn't start compared to when it did start, and being able to look for tell tale symptoms, myself as an armchair quarterbacks can only guess.