This summer, I stayed a few nights at
Underworld Tulum, and found the accommodations to be pretty solid for the price. As of a few months ago, they had 6 apartments, which could each be rented for $90 per night. The apartments have AC, contain a decent living room, kitchen with appliances, fridge, bedroom, and a shower with good water pressure and heat. There's also a little porch with a hammock, and a small shared pool area. There were the better part of a dozen dogs and cats bopping around, which were mostly friendly (they also operate a dog rescue onsite). Bikes can be borrowed to get to town.
I recommend giving Palma Central a try, half a mile from Underworld. It's basically a plaza with a bar, bench tables, and 5 or 10 food trucks. Nice place to go if different people in your group will want to eat different things. La Gordura was a great spot for tacos costras (tacos cooked with a seared cheese crust), and next door is El Grifo, a bar with a surprisingly good selection of craft beer.
Not sure if you have worked out your actual dives or not. But I will say that Lanny is an accomplished cave diver, and will provide guided cavern/cave dives at $200-$300 per day, depending on the specifics. I just did a day's worth of single-tank cavern dives with him, and had a great time. So I can't speak to the more advanced stuff, but Lanny is an instructor for full cave, multistage, CCR, and cave DPV. He's also just a nice guy who's fun to be around.
