I've sold/bought a few boats. 1/2 new on a lightly used boat is about right, 1/3 new on a more used but not damaged is still OK. Less than 1/3 new if the boat needs some repairs, but can still be brought back to life. These are all common rec boat prices.

Some still competitive racing boats may go for closer to new price, but I'm talking ~1k off of 4-5k$, so still a good discount from new. Plus you're dealing with getting on a waiting list for custom builders.