Have a look at XS-Ribs. New to the market and very reasonably priced. I've got one on order for May. The XS-600 would probably suit you best at £5495 for a new, ready-to-go rig. It has a high lift, wide beam hull so it may suit your engine(the reccommended hp is 75 -135). Just don't be expecting to go anywhere fast with 6 divers + kit with 90 hp on the back. Have a word with Laurence Lock at Barnet Marine, who knows a lot about rigging diving boats.
He has a used Tornado Viking 580 with a 115 Yamaha for £9995, set up for diving(bottle rack, etc.), so you could also consider trading in your engine against this boat or a Humber /Ribcraft with a bigger engine.