Bill if this is something that doesnt have to be done right away, I may be able to help you once we get over the busiest time of the year for us in Florida. The biggest attribute you should be certain is in your final solution is the flexibility of the software to work with your workflow, which may be similiar on the face of the scuba business, but once you get into your store may have its specifics that someone else doesnt understand unless they are standing in your store renting stuff.
Anyway, start defining your requirements, go from there, if you want to chat about any of it, chime me up. Who knows maybe I'll get some ideas to make my stuff even better
Schott