There are only 8 marine starters for GM bases motors
153 tooth flywheel (inline bolt pattern) or 168 tooth flywheel (straight across pattern) a direct drive or a PMGR. Then you have I/O vs Inboard (starter rotation)
Head to NAPA , bring your old starter and buy a marine replacement.
Most likely you have a direct drive, get a PMGR replacement
Do NOT use an automotive starter in a boat unless you want to meet Jesus