Jahan,
I was going to give a longer answer but my ISP keeps kicking me off in mid-message, so I 'll just say that a lot really depends on the person who takes you out on a given day. In most cases it won't be the owner but just a fellow who helps out. That person may be very solicitious of novice snorkelers ... or he may not be.
Wave chop, currents, tides vary from day to day and time to time, and there are areas of Hol Chan where the snorkeling can be a little dicey for kids or for the non-experienced snorkler or swimmer.
So best advice is to tell the guide that you are a novice and ask for his help and watchfulness. You're paying your money and deserve a safe trip. Also, spend some time before you go out on a boat snorkeling from the shore. You won't see as much but you'll get experience and confidence.
--Lan Sluder