Whilst in Tulum I visited a couple of the freshwater Cenotes for a very cold & 'refreshing' swim!
Cenotes are underground rivers where thousands of years ago the roof collapsed to make a sink hole full of freshwater. They are full of amazing wildlife but as they are caves they are so dark! You need a good underwater torch to explore properly and you have to be so careful as the ceilings are so low when you are swimming - sometimes less than an inch from the top of your head!
A few photos below, but they don't really do it justice to be honest. Our tourguide did take some good snaps on an underwater camera so I will post them up as and when they come through to show more of what it was like!