Re: fix up or sell as is
Generally speaking, you don't recover the cost of repairs, dollar for dollar. But having the repairs done can be the difference between selling at top retail or as a project/parts boat.
If a boat won't start because "all it needs is" a starter, then pay $100 and fix the starter, and sell the boat turn key. or don't, and sell it for $800 less as a project/risk boat.
Also, if the repairs are done, you have a faster sale, which might equate in savings if you have holding costs. For example, if you pay $75/month storage, a repair job + faster sale is worth $75 a month. If you are coming into a dead season, the faster sale is even more important (avoid your winterizing costs, too).
I say keep cosmetic costs to a minimum but make the boat look like it has been cared for. That's what smart boaters look for.