I really wanted to do this before we went to the beach while in California, but I didn't get around to it. I used the baking soda mixed with water method that they describe in Fun At Home but didn't have any shells handy at the time so I just made round forms with some sea creatures hidden in them. Then I added vinegar (with blue food coloring) to our spray bottles to spray on them and make them "fizzy".
Next time I'll use squirt bottles instead, our spray bottles really didn't release enough of the vinegar at once, although Bea still enjoyed seeing the animals emerge and the baking soda fizz! And Charlie took a turn at using the squirt bottle as well!
After we ran out of vinegar I added a little water and the kids had fun splashing around and making the sea creatures swim.
At one point Bea even dipped her toes into the water!
Great sensory bin for a hot summer day!