If you're new to carbs, (even if you're not) this is a great place to start http://www.bob2000.com/carb.htm
You can learn an awful lot about your tuning with nothing but a vacuum gauge and and tachometer. I too set my timing by feel (used the vacuum gauge) at idle and it was too far advanced. I think I'm at 9* now and the boat gained about 3mph top end. I had to up the curb idle to get it to idle nicely but it does. Based on the fact that it will idle nicer with a more advanced timing it sounds like the weights in your distributor may be sticking.