OK - haven't been to Nasau, but I wouldn't think it would have much in common with Costa Maya. Nassau has been a tourist destination for decades - Costa Maya is a brand new destination built in a jungle next to a tiny village.
Basically, the Mexican gov't has decided that Costa Maya would be a good location for the next Cancun. About 30 or 40 years ago, Cancun was an unheard of little village in Mexico - then their gov't did a study to determine the "best" location to develope a tourist destination and began to build/promote Cancun. Costa Maya is a similar idea but is only in its very first stages.
The port itself is a large pier where the cruiseships can dock without having to tender. They run a tram on the dock so you don't have to walk all the way in to the shore. At the shore they have built a "fake" village (It doesn't look at all authentic) which has most of the usuall cruise port stores. There is also a pool (maybe 2?) and they provide some "local" entertainment such as dancers or a donkey ride. There are restaurants and restrooms and many cruisers seem perfectly happy with this. There is a "beach" there also, but it looked pretty rocky.
One of the advantages of Costa Maya is that there are some neat excursions you can do from there. Good snorkeling should be possible since there is a famous reef offshore a little ways. There are good beaches up and down the coast - so excursions to those should be possible. There are mayan ruins in the area that are still being excavated - we took a trip to the Chocchoban ruins and it was one of the highlights of our trip. However, I am certain they aren't for everyone - if you aren't interested in archeaology or mayan history, then dealing with the longish bus/taxi ride, mosquitos, etc. wouldn't be worth it for you. Our group had a blast! Since Costa Maya is so new, the excursion options aren't all that developed yet - there aren't a lot of independent companies there or even that many for the cruise lines to hire!
About 2 miles from the Costa Maya port is a small village named Majahual. This small town had been a very isolated little place for a long time. Even now, their nearest "large" town for shopping is a couple hours away. It is rapidly growing and will certainly change over the next few years as the port becomes more popular. There is a beach along the front of the town and many slightly more adventurous cruisers have taken the taxi into town and enjoyed a day at that beach. There are restaurants setup along it and I have been told that as long as you buy something (some beer, lunch, etc.) that you are free to use their chairs, palapas, etc. I think there are even a little company or two that rent beach toys like waverunners, kayaks, etc.
This ended up being a little long, but I just don't think you can really compare Costa Maya and Nassau - they are in two different worlds right now. I hope this helped ....
pdarrah