Advise and help please?

I was wondering if any could give me some advise on what to do about my bathroom floor.
I have taken up the floor tiles due to a leak and discovered the actual floor lokks like it is a huge piece of MDF,it was really wet but I have managed to get it dry now.
Please could someone advise me on if I should cover the floor with a water sealent before laying tiles?
Also what are the best tiles?
Thanks

Answers:
Replace the subfloor that you can get too. It could develop mold. Also your floor will not be level if you try to lay new tile.

Other answers:
I Don't Know
I Don't Know
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