We're just back from the Cay Sal trip. They don't recommend it, but solo is allowed. I dove solo on the last afternoon as my buddy was finished by then(ear trouble). Since most of the moorings are near/on the divesites, it's easy. There's not really any supervision except they ensure that all divers signed out are back on board b4 they move the boat so you're free to dive how you want. The crew mostly assists you in getting in/out of the water, hands cameras down etc. and fills tanks continuously on the dive deck. With their divedeck design, it's really easy entry, you either GS off the sides - about a 3-4' drop, or walk down the dual stairs and drop into the water.
I only saw a DM in the water twice with a paying customer during the week - except for the shark feed during which all the DM's, the captain, the engineer and everybody are in the water together. The only other led dive was a deep drift dive in which a DM led, a DM followed, and they had the skiff out to collect strays if necessary.
Also a lot of the dives are in shallow water. If you have a full boat of divers, there's generally someone else within visual range since the viz is 2-3 times the depth. You won't have any problems buddying up during the week, at various times we dove 2, 3-with a single diver, 4, and once 8-10.
Note that unless you want to pay the 50% single supplement, they'll have you share a room with someone else if possible. The rooms are bigger than most diveboats, and beds are on platforms so you store collapsible luggage underneath. There's space to hang a few clothes, and 6 small shelves for your other stuff - for both of you. The sink is in the room and there's a separate pushbutton head/shower - great water pressure btw - and more hot water than you can use. Tall ceilings in the rooms also - I'm 6'3 and had no problems.
Really, all you want to bring are shorts/swimsuits, casual shirts etc. I didn't wear my shoes the whole week once onboard. The rooms are pretty humid if you want to keep them cold, you run the a/c on low all the time so your clothes are damp all week - but a few minutes on the sundeck and they're fine. It's a steel boat so there's some condensation issues, the crew will take care of it as soon as you mention it.
Questions?
I recommend Nekton highly.
