5. WHITE LIES. Let him know that you're human, too. You also have obligations that need to be met. Try a variation on this: "I really need the boat back soon because I'm taking my son's Cub Scout troop out for a day on the water. I don't want to let them down by not having the boat ready. We were planning on going out next Saturday at 10 a.m." Remember to set a specific time and have him agree to it - out loud.
6. CERTIFIED? OR CERTIFIABLE? Probe a bit before you bring your boat in for service. For instance, are the mechanics employed there certified to work on the engine you have? Just because there's a certified-mechanic sign hanging over the door, don't assume the certification was done recently. That sign could be from 1905. Is the shop a family-owned operation or part of chain? This information will come in handy if you need to log a complaint.
7. !@#&! We hope it won't come to this, but if it's necessary - bitch, moan, and complain. Rage like a rabid raccoon if you have to, then find someone who can make managerial decisions and threaten to take your business elsewhere: "Sir, I don't want to be a pain in the ass, but I need this repair done right. If you can't do it for me, I'll give my business to some other shop, although I'd rather not." There's a multitude of psychological games being played here. Try it.