I tried taking a picture of this, but it didn't turn out too well. I'll do my best to describe it.
Foodsaver (or ziploc bags and packaging tape)
Blue hair gel (cheap as you can find)
Plastic fish (small party favor size)
1. Place fish and gel in Foodsaver bag. Make bag any size you want. Mine is 6 x 10.
2. Seal up with a little air still inside.
**If you don't have a Foodsaver, you can use a ziploc bag and lots of packaging tape to seal it securely.**
It will look like the fish are in water. Toddlers love to squish the bag and find the fish. Very easy and not too expensive. Other items that could be used are bugs. Be creative.