Water and everything that comes with it!

It would be super nice to build submarines or boats. With new blocks, like underwater weapons, sails, pression sensors, periscopes, raft rope, grapples, etc. The blocks would either float or sink, depending on their weight/size, and the sum of it would make the entire machine float or sink.

There could be underwater/aquatic missions, with maybe merpeople, pirates, and aquatic animals (like mounted or armored whales, sharks, ducks, eels, etc.). Waves, storms and tsunamis, along with tides and weird, strong wind could make it more difficult. Also, customizable flags to put on our boats would be awesome!