Nothing too major:

(1) Don't go out without either your Towboat US ID or a credit card.
(2) If you see somebody building a finger pier next to your boat, go back after them and see if they messed with your tie lines. (17 those ass holes; cost me $1000 dollars in fiber glass repair; probably the biggest mistake I've made that bit me).
(3) Always have a good knife on a boat. A Swiss army knife is not sufficient. Get something big and with teeth to cut through lines.
(4) Never let anybody else put the plug in, hook up your boat, hook up your trailer, trailer your boat, basically anything that is vital without double checking behind them.
(5) If your cranking off two batteries, periodically check both so you don't end up off shore with one dead battery.
(6) Never let anybody else check to see if your lines are in before cranking up. Twice I've had to cut rope out of the motor because somebody was looking at a line in teh water while they told me all the lines were in. Neither were dubmasses either. Would have included this with (4) except it's so damn unexpected I thought it deserved its own item.
(7) Never pull up anchor until your engine is cranked and in gear.
(8) Never trust a boat gas gauge, even if you are giving it a half a tank margin of error. Ran out of gas once after the gas gauge went from full to 3/4 full.

The biggest dumb thing I've done repeatedly is do things beyond my comfort/skill level. Looking back I did some things that I'm lucky didn't end badly. I'm much more cautious now.