For serious cruising I wouldn't own anything other than a hard bottom inflatable, unless I had the room to carry a solid boat. After 8 years down island our RIB's bottom had been so abused it looked like someone had drug it down the road behind a pickup, but it just kept running long after an inflatable floor would have died. We actually started with a soft bottom, and ripped a unreparable gash in it six months into the voyage. Something about a coral finger as we pulled it up on the beach.
I would also really recommend getting as large an engine as you can. You won't be anchoring near most dive sites, and while it is nice to think you won't mind the slow trip to them, when those trips are over even remotely open water a fast
dinghy is a safety issue as well, since getting back before storm break can be critical. It also means you will be much more likely to take dives since it changes a Day trip to a couple of hours. Finally in the event that you need to use the
dinghy as a tow boat, which we had to do a number of times, a larger engine will make a world of difference.
Finally carry the biggest boat you possibly can. While island cruising they are often the only way to get water, provisions,
fuel, people, parts, ect onto your boat. A small dingy that requires multiple trips makes all of these thing much more difficult. I remember having to reprovision 250 gallons of water, and 100 gallons of
fuel by
dinghy. As is it took us all day, in a smaller
dinghy it could have taken 3 days easily.