Before I bought my 6.5m I was looking at a 4.3m Quicksilver HD. This would have given me loads of space for diving from with 4 of us on board. If you want to just transport gear and divers around you could probably get away with a 3.5-4m craft. I did dive from a 3.1 Zodiac but there were only 2 of us and it was very full with all the dive kit so couldn't have got 1 more on, and we had to throw the kit in the water and put it on there.
Look for something 3.5-4m that you can get a 15-20hp engine and you'll be sorted