Can't help you with anything recent, but we were down on that area (Xcalak) two years ago and the diving was great. I don't know about Mahahual area, but in Xcalak, there was no shore diving. The area inside the reef was quite shallow, the reef was a long ways out from shore and had a lot of wave break. The areas aren't that far apart, so I think it's about the same.
The diving is alot like what you would get diving Ambergris Caye, only 30 miles or so apart. We dove a lot of slot canyons, drop down on the reef, find a slot, drop down, swim to the end, swim up and over or around to the next slot, repeat. Max depth was about 80', some times not more then 60'. Boat live drifted above us, and we ascended when we needed to. Tons of life and the reefs were in great shape. Lots of color. Highlight was a free-swimming moray that was probably 6' long and as big around as my thigh. Came out on a sand flat one day and there were six huge southern rays hunting.
We dove with XTC diving out of Xcalak. One of the owners of the resort we stayed at dove with us and guided. Basically used the shop for a boat ride and tanks. There was never anyone within eyesight on the surface. We were always the only ones diving on a site.
When we were researching for our trip I got the impression that the shops in Mahahaul catered to the cruise ships which it sounds like you might be doing. I also got the impression that there was a pissing contest going on between a couple of the shops, which was somewhat verified by the folks in the area I dove in. Some of the diving reviews I saw from Mahahual were less then stellar with the dive times being restricted, and folks not getting picked up for dives, and general feeling of cattle boat diving. All of this kind of steered us towards Xcalak, and we were very happy with our stay there.
I'm not sure how the area has faired since last years hurricane, as far as shops being open, infastructure being re-built, or reef condition. You might check on Cruise Critic, and see what others impressions are. The day we drove up to Mahahual to check it out, there were no ships in port, and it seemed peaceful and laid back, but I bet it could be pretty busy with a ship or two in port.