I'm 59 years old and have never been without some type of boat ever since I was 15. Other than sinking my truck, I'm guilty of about every mistake mentioned in this thread. The best advise I can give, is to take your time, don't let others make you feel rushed and to keep your wits about you. Most of my mistakes have occurred when I am being rushed or are distracted from my normal routine.
Something I haven't seen mentioned, is to get a good lanyard for your sunglasses. Cant tell you how many pairs myself or others in my boat have lost due to turning around and looking behind the boat while cruising down the lake. I always keep a couple of pair of cheap ones in the glove box for just such occasions.