Mdf worktop with tiles

Hi,

I'm looking to build a worktop using granite tiles. I was going to lay them on marine ply because it might get wet/ damp. Just come back from Travis P and having got a quote for the stuff i'm now thinking water resistant MDF. Does anyone use this for applications like this? I'm thinking two layers of 18mm to make sure there is no flex. Any advice gratefully appreciated.

Cheers
Matt