Do yourself a favor and book your snorkel trips through the resort. They'll call the tour company and make the arrangements. If you want, some resorts will even make the payments direct to the operator and add the amount onto your room. Do you really want to trust someone you just encountered on the beach? You're safer checking out the guides that are listed on this website (//ambergriscaye.com/pages/town/dive.html) and knowing that they're reputable.
We've done trips with 'lil Alphonse, Searious and Seaduced and had great times. With some visitors, Alphonse is becoming a bit of a local legend. He is excellent with the marine life, he loves the sea and he knows his stuff. He's patient with beginners and knows enough to give more freedom to experienced folks. However, if you do a Hol Chan snorkel, you can expect every guide to be more regimented - you'll probably be in a group and it's a national preserve that they take good care of.
In addition to Hol Chan, there are several good snorkel spots north of San Pedro, such as Tres Cocos and Mexico Rocks. You might also want to consider taking the water taxi to Caye Caulker and going snorkeling from there - you'll see a number of tour shops right along the beach.
For the best snorkeling, you will absolutely want to go with a tour operator - they're the only way to get out to the reef. If you're adventurous, try a night snorkel (we've been twice with Alphonse) at Hol Chan - it's a completely different trip.
Finally... although I could forego the manatee watching - generally murky water, less than clear views of the manatees - most manatee watching trips also go to Goff's Caye and then to another snorkel spot near Caulker. Goff's Caye and Caulker are worth the trip (IMHO) without the manatees. Take a look at the website for some of the tour companies and check out their trip listings for more information.
Bottom line, you're gonna have a great time.