p.s. Read the comments to learn how to make it a floating tray, a lantern, a water distiller, an electricity generator (solar or wave motion), a clock, a raft, a floating island, a fishing rope...
Thanks for all your great ideas, Jeff-O, iSmack, Btop and all the others.
Step 1: What you need
-A board of wood or plastic at least 0.5 inch thick (no fiberboard)
-A plastic rope
-16 Pan Head screws
-16 water bottles and their caps
-Paint (the color you want)
-A piece of flexible foam (1/2 inch thick and more), like a square of Play Mat (I took my little sister's)
-Teflon tape or silicone tube (optional)
-A hot glue gun