We were in Chankanab in May and it was $19/pp to go in. There are coupons on the internet. The cab fare is usually $10/one way. The entrance fee does not include snorkeling gear or the dolphins.
Belize and Costa Maya have some fantastic snorkeling. We did excursions through the ship in both ports that were great. Mahual is not far from the pier, you could actually walk. There is a Mamasita's in the terminal area with a swim up bar and plenty of shopping. I was going to take the tender back to shop some in Belize, but the weather got too nasty for the tenders to run. I had been in Belize before but had only snorkeled, so I wanted to check out the shopping. Maybe next time
Roatan's Mahogany Bay is a great beach day, there is shopping, and they have a gelato shop with really excellent gelato. If you were docking in Coxen Hole, I'd suggest going outside of the terminal area for shopping. The same goods outside are less than half the price. Mahogany does not have a local shopping area that I could see.
The water on board the ship is as pure as any bottled water you could purchase. If you want to bring a few bottles with you, you could fill those up before going ashore. Please use a cup/glass to fill your bottles, though. There are signs posted, which people ignore, to not fill your bottles in the buffet area. Really, it's much healthier if you just use a glass and pour it in. The drink ware has been sanitized where as the mouth on a bottle has not. But, there are always those who think the rules are for everyone else but them
You're going to have a marvelous time. The Dream is a great ship!