We stayed at Costa Maya Resort in February. I have posted photos of the resort in the trip report section of this message board. Needless to say, we will never stay there again for reasons I will not go into. Luckily we were on A.C. two weeks and for one of the weeks we stayed close to town.
That being said, it is a nice resort as far as the grounds are concerned and our room was very nice. There is nothing to do there but lay on the beach, sit at the pool or stare at the beautiful ocean, which is great if that's what you want. If you want to get away from everyone and everything, it's the perect location.
However, if you like activity, being busy or partying in the evenings and at night, you will be bored to tears. It's REALLY, REALLY quiet there. You should be able to keep busy during the day doing tours, though; however, they only offer each tour on a specific day and they do fill up. So if you miss a tour one day, it probably won't be offered again.
There is only 1 restaurant nearby but I forgot what it's called and you have to walk a ways to get to it. You will be stuck eating at Costa Maya's restaurant most of the time or cooking in your room.
Yes, if you want to go to town, you have to take a ferry. We had to pay $10 per person one-way, not $5. It runs every day and has several departure and return times starting mid-morning and going later into the evening. It comes and goes every hour and a half or so.
The resort does offer a shuttle 2 times a week for $5 each way, but it leaves at like 1:30 or somewhere in there and comes back around 5 or 5:30. Not a very flexible schedule.
They also have a complimentary shuttle on Saturday that goes to town but I don't remember the time it leaves and comes back. It's a pain trying to schedule everything around their shuttles and it never worked with our schedule so we always paid for the $10 one-way ferry. Maybe you'll have better luck.
A golf cart at Costa Maya is not feasible except for maybe one day for exploring. We did rent bikes one day and rode to town and spent the day in town. If you like biking, it's not bad at all. If you're out of shape, probably not a good idea.