First we did a "Will it Float?" test with several different items to see which materials would be best for building our boats. Then we used tape to create several different boats. Here's what ours looked like.
Next, we filled the tub and added a few drops of food coloring, because who doesn't like a green bath?! Plus it's Christmas time. Some boats floated better than others and some fell apart, but Rowin was in there for a good half an hour! He didn't want to come out until he was shivering and wrinkly.:)
We will definitely do this again. He loved it! And I think this would be fun for any age. If you have older kids, you could make it more of a science experiment. Have them make predictions about which materials will sink or float. Discuss their schema (what they already know) about boats to help them build one that will float. Discuss buoyancy. Have your child rebuild a model that doesn't float, using what he's observed from boats that do float....can you tell I'm a dorky teacher-brain mom? :)