Hey Amanda. Yes, our rainout option was in the basement... HOWEVER that was for the Akumal side. Who knows what it is on the Tulum side.
We had our welcome dinner in the buffet. They blocked off a section for us and it was perfectly fine.
Honestly, if it rains don't worry about being in the basement. It'll still be beautiful and you'll be surrounded by your closest family and friends. Not to mention, you don't need to STAY in the basement. Have your rain plan be to eat dinner down there, toast, maybe do an hour of dj with your first dance, and stuff like that... but then have everyone go to the bars with live music Could that be an option?