I've never been but from what I have read while researching, it is not the turquoise blue water you would see on the Cancun hotel zone. The water is more greenish blue in Playa Mujeres but does seem to be much more calm than the hotel zone. Sometimes there can be seagrass but assuming the resort does a good job raking and cleaning, it shouldn't be a problem. Can't control mother nature so no worries. To get a better idea of the beach conditions, check out photos on Tripadvisor for Excellence Playa Mujeres or The Beloved Hotel. They are on the same strip of beach.
I'm so glad to hear I'm not the only one who chose a resort that is not open yet. My date is April 19, 2015. The wedding coordinator, Claudia Cantillo, has been pleasant to work with so far. She used to be the WC at Dreams Cancun. <-discovered that on this forum. Happy planning Megwade!