That depends on a lot of things. If you are just looking for a dive boat I might not suggest a RHIB at all.
Here are some of the factors that you should consider though:
Cost: obviously this is going to be a driving factor but your budget is going to limit what you can get.
Hull floorplan: (not sure what to call this) but if you look at a boat from above, faster boats designed to run in heavier seas are going to have a pointier bow, Ive been in some RHIBs where the front 4 feet of the boat are useless because they are only wide enough for a guy to stand in, you cant store much gear or get dressed in this area
Hull profile/hull type: A rigid fiberglass/aluminum deep vee hull is going to provide you with more freeboard, it will also provide you with more stability at speed, better handling in rough water, but it will make entries and exits more difficult in dive gear. Compare this to a zodiac with a flat or shallower vee hull, the pontoons will be at water level making it very easy to get in/out
Console: If you have a console it will be nicer to drive, you can have an awning, electronics, etc, but it will take up a LOT of valuable floor space that could otherwise be used to put people and their gear.
Inboard/Outboard: Inboards can be cheaper/easier to maintain (require a solid hull), however outboards can be easily swapped, and you can mount two for redundancy, etc.
RHIBs are generally not (in my opinion) the best boats for diving. Government agencies use them because cost is not a big deal and they are very seaworthy. If you are an individual with a normal person budget I would recommend a medium sized zodiac type inflatable with a flat/rigid insert type floor. Coupled with a small outboard motor, this boat is easy to store, easy to transport, can be used for diving and for family camping, etc. They are very versatile boats. But if I was buying a dive specific boat, I would look for some sort of aluminum work boat with a pair of outboard motors, benches, possibly a cabin, a helm, a dive door and a solid dive ladder, or if you dont intend on being in rough water, landing craft style aluminum boats. Its great to be able to just swim onto the ramp!
Anyway, I hope this helps, good luck.
Maybe look into something like a Zodiac Bombard Commando C5 (for say 4 divers plus a boat driver) and then around a 25-40hp outboard. Probably your most affordable option, and you dont need to go name brand like Zodiac, there are lots of manufacturers who sell boats of similar quality for less.