While the water is good in some situations, it creates a mess and can be dangerous, in that if the bottom of the mat isn't wet enough, the mat can still slide when it is assumed it won't move at all...or it won't move when it needs to.
You may want to look at how you are using the mats first. Water is not recommended by any equipment company for a reason...
Stacking larger mats on smaller ones, creating uneven surfaces, or training skills which are supposed to "drop"onto mats (flyaways, dismounts off beam) incorrectly can cause the mats to slide, when the mat itself isn't really the problem.
I have had kids whose mat "slides" on skills, and I let it slide and explain to them that they aren't supposed to hit the mat at a 45 degree angle upon landing.
When jumping onto a panel mat, the athlete should jump up onto the middle of the mat. Otherwise it should slide due to simple physics, and it's important for the kids to understand how the landing surface reacts.
I just don't believe that changing the environment to make the activity easier is always the answer.