The cruise op on Roatan is probably
Anthony's Key resort (they are for some cruiselines) Since they're on the north side and the port (Coxen Hole) is on the south, they shuttle you both ways. Spooky Channel in that area is one of the best Roatan dives. There's also the Dolphin Dives there. They do have a lot of boats/guests at times but we went there one morning and there were just 5 of us on the boat.
Cocoview (
Dockside Dive Center) on the South side (10mins. from the port in Coxen Hole) also accommodates cruise divers but I've read they require a checkout dive first.
Advantage for them is one of the few good shoredives off their property and close access to Mary's Place.
Another option is the
Waihuka Shark Dive, either booked through your ship or via one of the booking agencies - they don't take direct bookings. It's a fairly advanced dive sometimes.
Coconut Tree Divers in the West End might be another option - they do a two tank that would be back b4 1PM. You'd have to take a cab there ($10pp - negotiate in advance) and be there by 9AM. It's about 15-20mins. from the port depending on traffic. Cabs congregate in front of CTD and you'd have a lot of options for lunch and shopping in that area if you have time.
On Cayman, the cruiselines use Don Fosters in town. Not what you want, they have a
fleet of bigger boats to accommodate all the traffic they get being 100 yds. south of the cruise dock.
Most of the diveops on Cayman are used to picking people up all up and down the West End. All of these advertise 8 divers max: Neptune's Divers, Off the Wall, Wall to Wall, etc. This page links to all 3 and more options:
Cayman Islands - Grand Cayman
fwiw, everybody I mentioned provides tanks/weights. It's pretty much a given.