I work in the dive industry in the states. It's not unusual to pay $60-70 for a two tank charter, no gear provided. Beautiful diving but cold water and gear intensive. I don't think it would be unheard of to charge similar rates for diving in Belize. When I'm planning a dive trip for my customers cost is of course a concern but more importantly is the destination. My store has a trip planned for AC next month. When planning this trip I asked my customers where they would like to go. Reputation more than cost helps me decide. In my experience, if you show people a good time cost becomes secondary (for most folks!)
So, I say, go ahead and raise your prices to stay competetive and be profitable. You may be surprised to see other shops follow suit.