Sorry but I have to disagree with most of the info the people (divers?) above have given you. I was at the Blue Hole a month ago, snorkeling, and it was incredible! I was staying 20 minutes away by boat - so in quite a different position than having to take the 2 hour boat trip each way - but if you really want to make this once in a lifetime experience - take 2 Bonines and go for it! I've snorkeled all over the world, including the local AC areas - and have never seen such live coral and variety fish. The divers all felt it was anti-climactic, but a must-dive. I was on the boat with them for a week, and saw a lot more at almost every spot.
It was completely calm inside the rim.
And, most day trips stop at Half Moon Caye - 2 more great snorkeling sites.
If you go, jump in the water the minute the boat moors in the Blue Hole, because the divers are down for a short period - and you really don't get a lot of snorkeling time.