Hello,
If your starter is turning as should but does not turn over the motor then most likely you have a bad bendix which is located inside the front cover. Or the splines are stripped on the starter. Best way to tell is remove the starter and inspect the splines if ok and not worn down/stripped then confirm it is spinning and if starter is ok then replace the bendix which will require removing the front cover off of the engine.
Thanks,
Joe
Marine Mechanic
ASE, electronics, Marine inboards and electronics, automotive drivability, all around repair tech,