I did exactly what you propose- booked with West End Divers while we were on Roatan for a cruise. And for the same reason - the cruise excursion had booked fully. We had a great experience: they provided a driver both ways from the port, the dives were well run, and they even arranged for us to have some extra island time post dive to shop West End and grab something to eat before the car took us back to ship. There were a couple less experienced divers in the group who used up their air very quickly - but they were able to return them to the boat and allow rest of us to continue.