Are you asking about the Grand Mayan/Mayan Palace Riviera Maya RCI #6483 or 6973? If so, that's where we're headed this April. If not, just ignore the rest of my post
These are basically the same property. My understanding is that the GM units are a little larger, with more upscale furnishings and perhaps a better location. At this property there is only one (huge) pool that is used by both GM & MP guests.
From what I've read RCI is moving some people who have GM confirmations into MP units. Personally, this would not bother me and I'm booked into an MP unit anyway so hopefully won't have any problem.
However, I think at some other GM/MP locations the guests are more segregated and the GM has nicer grounds, pools and such that "regular" MP guests are not allowed to use. This seems to be where most of the complaints are coming from, and I can't say I blame the exchangers in this case either.
If you are booked into the Riviera Maya property I'm sure you'll enjoy either one too, and shouldn't worry. I've not heard about a water park there, but I admit I don't have the most uptodate knowledge of the resort!