If you're looking to dive around Marathon, I've dived with Abyss in Marathon twice; once in 2008 and once a couple of months ago. I'm not a very experienced diver, so I don't have a lot of dive ops to compare them to, but on the other hand I'm fairly picky about service and boat stuff.
I found them to be really squared away in their operations, and also very friendly and helpful. On one of my trips the conditions weren't great, and they took some time to try to find the best spot - including contacting other boats in the area and moving once after the DM hopped over the side and checked the conditions. As it turned out, nothing helped that day, but they did try.
The boat is a 6-pack. The Captain works it from up on the fly-bridge, so you have an inside (open at the aft end but with a roof and three walls) seating area for 6 people, plus other seating in the open cockpit aft. Dives were for about an hour or until 500 psi. Procedures were all well and clearly explained topside, but then you could go your own way once you were in the water (unless you hired a guide, which I did in 2008 when I was rusty and did not have a buddy).
They're up on the north (east) end of town, on the ocean side. The boat is right on the premises with a nice, quiet palapa area to sit while waiting for the boat. They set the tanks up on the dock and then had us fit our gear to them, after which they loaded them onto the boat. That seemed kind of nice, vs. staggering around trying to do it aboard (seas were a bit rough that day). They also had separate rinse tanks for "face" gear (regs and etc.) and the rest of the gear, with different sanitizers in it. Made me feel a little bit better than usual about my rental reg.
I'd characterize the overall feel as a bit more mature vs. being with the cool kids (that said, the DM was probably 22, but I'm talking about the general impression).
When I was there in 2008 I was with a group of friends that included 3 divers and 2 snorkelers and they accommodated us nicely, making it easy for us to spend time together and yet still get in enjoyable dives/snorkels.
B.