Sunken Treasure Sand and Water Table Idea

In advance, collect small rocks.  Lay them out on newspaper and spray paint
them gold.  After them have dried, flip the rocks and paint the other side. 
Put the rocks along with a few plastic ships in the sand and water table. 
Have the children find the pieces of treasure.  You can also have them put pieces of gold onto the ships and see if they can get the ships to sink.
This is a good a easy simple science about sinking and floating.  While you are playing talk to the children and explain that many pirate ships actually sank.

You could also use these pieces of gold and bury the treasure in the sand.  You could give the digger directions on how to find the treasure.