I am putting down a Fake wood laminate floating floor on my concrete floor in the basement. Can I put polyurethane after installation to protect from water.

I wouldn't becuase most of them are treated to be water resistant and the polyurethan may not adhere to the already slick/smoth surface of the laminte. Also, it could wind up looking super shiny and could be harder to clean in general. One of the nice things about the floating laminate floors is that if a board gets damaged you can pop off your wall base, pull up some planks, and replace the damaged piece. If you seal the top, you wouldn't have that option!

You will need a plastic sheeting to lay down and tape the seams.The laminate will lay on top of that.Outside of that the best thing you can do is during installation apply a bead of glue on each side that when dry will keep moisture from penetrating between the boards.