You're on the right track size-wise. Any more than 4 guests and you're asking for conflicts and a LOT of work. Don't plan on getting rich - but for help in offsetting the cost of ownership it works. The hard part is in getting the guests in the first place. Taking care of them and showing them a good time is easy! You don't need a new boat - mine is 31 - but you MUST maintain it properly and make certain you have good gear and the proper "toys" and connections for fresh seafood and SCUBA if they want it. I've been at it for 6 years now and am having fun ... but for me it is strictly a source of additional funds, not a sole source of income ... and my boat is paid for.
Feel free to drop me a
line if you have specific questions.