As a kid I loved the giant boat hull piece because it was sealed and actually floated. This in combination with some larger pylon-type pieces from the Star Wars set meant you could build floating cities and vehicles and such and mess with them in the kitchen sink.

I wish I had hobbies as cheap as LEGO now...

They still make "boats that really float" today: https://www.lego.com/en-us/product/arctic-explorer-ship-6036...