When we did the Blue Hole trip there was a snorkelling Option. It was not, in my humble opinion, worth the length of the trip though. The Blue hole is not a great dive. It is a deep dive but that does not make it a great dive. The dives afterwards are nice but personally speaking, the local diving if you have just certified is more than enough. Diving is a nice slow and relaxing thing to do but boy, does it wipe you out!
On the roads, last time I was there the roads up north were still rough, the water taxi was the best way to go or a long walk on the beach with a few stops at the local hostelries
Boat rentals I will leave to a more local person to answer.
Money? It really depends on where you go and what you do. Rent a golf cart and your outgoings will soar. Eat at the more local restaurants and you'll have some great food but if you go to the resorts and 'better' (I may take argument with that definition) places you'll spend a lot more.
Most places have web sites but not all show certification prices, check trip advisor as well. We will probably try Chuck and Robbies this year - we have dropped Amigos del Mar due to very poor front of house service - that's my admittedly biased opinion.
At the risk of sounding like a complete tourist (I am though) we have been to Belize multiple times and still find it tough to beat going into town and sitting in the Reef or Fidos while we plan our next move.
Have fun!
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