I did if after discussions on here - I put my 36 x24in mat in the tub - of course it had to curl up the sides, which worried me a bit.
I filled up with luke warm water, right to the edges of the mat, and left it overnight. I was amazed to see that the water level had dropped about 2in at that point. I topped it up again to where it was before, and left so it had been in there a total of 24 hours.
Took it out, dried it off, and put it back on my table up the other way to give it a chance to flatten down again. It didn't completely get rid of the deeper cuts on the board, but it is a darn site better than before the treatment.