Our newest sensory box is an underwater adventure. The base is blue water beads. (We usually get them from Amazon or Wal-Mart) They are located in the floral section of stores. Most people use water beads for vase fillers when making arrangements.
The other pieces are random sea creatures we found at different stores or had around the house. My mother-in-law even found some ocean stickers from Oriental Trading that the boys have just put around the outside of the box.
My only suggestion for this box is to use it outside. The water beads do bounce, so they make a little bit of a mess if they get out of the box. This would be a great summer box! The beads last for a month or so if they are sealed up. When you are done, they can be placed in a flower garden for moisture.