First we filled a bin with the craft sand. You're going to want to specifically use craft sand for this activity to get the right consistency, but it's totally inexpensive and a lot of fun anyway.
Next, we added some water until the craft sand was outnumbered by water.
The cool part about craft sand is that it doesn't absorb water and get all muddy like regular sand. It simply gets wet and also separates nicely from the water. Ethan liked when we lifted up the bin and saw the marbles and sea glass I'd dropped in sitting at the bottom of the water. He used his hands and a measuring scoop to reach in and scoop them out.
And, like he had planned from the get go, Spiderman enjoyed swimming in our sensory swamp, too.