I think you must have the V-drive, so the starter is on the port side of the engine, near the bottom and forward. Disconnect the battery cables at the battery, then you can undo the large cable on the starter and one (or 2) small wires. There will likely be a medium size alternator wire paired with the large cable. The starter is held in with 2 or 3 long bolts.

That's how easy it is.

With your boat year and engine option you can pick up a new one from most parts houses. I don't know the pn.

As for your diagnosis...there are 3 reasons for your symptoms. Most common is a bad battery. Second is corroded connections on the large cable between the battery and starter. Finally is the starter itself. I usually diagnose in that order.