I have lots of thoughts on this, but the short answer for now is:
Snorkel off shore (we did Ulua Beach & Wailea Beach), and save the boat trip for the whale watching (a MUST DO, at least when the whales are in town!).
Molokini was less than stellar as far as snorkeling, BUT...
As far as an experience on a boat and an adventure, the Four Winds II trip was awesome. We saw tons of whales, a few dophins and turtles. The crew was great, food was great, and the boys got to drive the boat! The snorkel end of it was less than great, but we'd just had tons of wind, and maybe it wasn't the best day to be there.
We rented Boogy boards and snorkel gear for the week at the Maui Dive Shop. There are tons of locations, and they were quite knowledgeable. For gear for the entire family for the week, it ran us under $100, and was well worth just having it in the car, and ready for adventure at a moment's notice.