Mast Step

Many have slightly different steps: mine is flat, square, with a upward protrusion that mast sits on.  Some people have re-inforced area under step with beams and even posts...whatever you need to do.  Water can leak into core from mast wiring or other items...be vigilant in looking at  a boat. 
I prefer deck-stepped, as if it goes over, you don't have a honking big stump smashing things around below because it has come loose!  Yes, you may not have a stub left to jury rig, but our boats are so fine through the water that you could use a spinnaker pole as a mast if you had to! ;-) Oh, hopefully the boom and sails didn't go too! Oh dear, could be quite the adventure!