The snorkel spot on Kaanapali is Black Rock down by the Sheraton. One of the best in Hawaii.
I don't float well and really like something to help. If you get the blow-up vest get the newer kind that is more like a jacket, which will stay put better on you than the Mae West vest that rides up. If you want to avoid cost till you decide if you like snorkeling, a kick board works well without the bulk of a boogie board.
Black Rock once you get out a very short ways is a vertical face over deep water, which makes it easier. The flotation assist is needed more to keep you up off a shallow reef. If its a really calm day you can go around the end of Black Rock to a notched-in v in the rock that is shallow but has really great fish color. Don't try if the sea is not very smooth.
Don't get the biggest fins you see, and they may frustrate more than help off the beach in a calm area.