You don't need another opinion that you don't need to rent a car, but here it is anyway...

If you are mobile enough you could even get away with no taxis, even to La Perlita or El Moro (they are a bit of a walk, but 20 -30 minutes). But they're cheap enough at any rate. And a $35 rental from Margarita at ISIS will get you a car for a day touring the island if you like.
I'll also weigh in on Casa Mex, it's very centrally located in town, which can be good or bad depending. I've stayed there 4 or 5 times, breakfast is nice, rooms are clean, but -- and this is just IMO -- unless you are on the top floor, the rooms, at least the ones i've been in, are sort of cavelike due to the architectural walls. We've stayed both on the front ocean side and the back town side (ugh, don't even). There are also not a lot of places to lay around and relax post dive other than the bed... I don't mean sunbathing. BXTSea picks up (or used to at least) at the pier in front of Bahia Suites so it's about a 2 block walk.
A few years back we found Blue Angel, and now it honestly feels like more of a tropical location. Very nice grounds with a lawn and chairs, hammocks, a nice pool with a little shade, and a downstairs lobby area that is very nice to hang out in post dive. There is a restaurant there as well that is in our opinion very good for a few meals. It's walking distance to town - about a mile -- or a very cheap cab ride, 70 pesos depending on the driver :^....
BXTSea picks up right at their pier. ETA there is also a decent shore dive at BA with an on site operator for tanks and weights.
Anyway, just my .02 on an alternative to Casa Mex. Both are very nice, but both have different vibes and offer a different atmosphere, all depends on what you are looking for from a vacation.