Diane, we had Giovanni and Christian for the whole day. If you do a private thing with them like we did, you get them for the day, and they can customize the trip to your likings.
We met them at their headquarters (a short walking distance from the pier where the tenders let you off), got out equipment (which was included, although I used my own), and headed out and up the Belize River.
It is quite a distance from the river to where we went snorkeling (at the barrier reef), but the ride out there was beautiful! I'm told that you can see bottlenose dolphins enroute, but we did not see any that day.
After snorkeling, they took us directly back to the ship in the boat which we used. We had about an hour to spare before we set sail for Roatan. The SeaSports people brougut us right up to the gangway, we just stepped off their boat and onto the Paradise. The "speedboat" that I referred to earlier is the tender. They travelled really fast to get to shore, almost like they were pulling water skiiers! Although it was exciting, we were not prepared for that trip!!
We did book in advance. Linda from SeaSports handled everything for us and we communicated by fax and email. It was pretty easy. BTW, if your ship does not make it to Belize, don't worry, you will not be charged. This happened on the trip before ours (May 4th sailing) because the waters were a little too choppy to drop anchor.
Excursion, if you are interested in getting a group together to do the snorkeling, maybe you want to post your information here and get some takers. Tall them what ship you're on If it's the Paradise and you don't do anything till you sail, you'll only have a day (Mondays are At Sea) to recruit.
In any case, hope this helps, and I'll be glad to answer anything else.