we dive a 23ft keywest in the keys, a 23ft proline up here, and a 19 sailfish here. the 19 id only go out with 4 divers and that is loading the boat heavy where 3ft or less seas is the absolute worst we would want. the 23 proline and keywest work well for 4ft or less with 5 people but would troll at 8ft on either. between them all the KW is not a dry boat you get soaked if there is any chop. The main thing in finding a boat good for fishing is open deck space, bait lockers, rod holders, high gunnals. For diving you would want tons of room on the back of the boat, a notched transom is very nice for diving, but fishing full transom is better. a nice dive platform with a ladder is a great convenience too.
When diving on a typical private boat they can be trickey getting out of the water where it limits the height of the seas you would want to go out in. Just look at some of the options like tubs for keeping tanks contained, or tank racks. types of leaning post/seat options and rocket launchers for rods, and see what you could come up with. on the proline, we use a marine cooler to sit on, prop tank in corner with BC attached then sit down and strap up. There are various things to consider to better accomidate diving/ fishing you just have to be creative