All you need is some kool-aid packets, ice trays or even small bowls to make bigger boats. Then place sticks or straws in them to freeze and add the sails once the water is frozen.
The to make your stream just get some aluminum foil and add water. We added some rocks to ours. (You can build it however you want)
The kids had fun using their water squirt toys, filling the stream and melting the ice. Fun way to play and stay cool in the hot summer weather.