I am with you Steve, I am have been out with Searious 3 times and each time was better than the last. The Manatee is the best, like he said, you do two snorkel stops, lunch(hopefully Willie makes his awesome brownies) at Geoffs Caye and look for Manatees and Dolphins. I did AltunHa/Maruba twice. That was also very nice. You take a boat to Bomba, a little city, then get on a bus and drive to the ruins, then you stay there for a while, then you go to Maruba Spa and have lunch and either get a massage, horseback ride,lounge by the pool or drink Viper rum.
Willie is the best tour guide, he knows about the sea and the ruins.
Another great trip is the catamaran with Unity Tours, a group of brothers, that is also two snorkel stops, Hol Chan and Shark Ray Alley and lunch on Caye Caulker,then you sail back, it is a very relaxing trip.They really know the sea and they are a great group of guys to go with.
You will have a blast with both Searious, no matter what guide you get,but Willies is the best, and with Unity, those guys are great.
Have fun,those are my top three choices:
Searious Adventures-Manatee and AltunHa/Maruba and the catamaran with Unity Tours.