This got submitted as a launcher issue before, but is a game issue. (Sorry)

The boat sinks. I'd expect it to sink in waterfalls, but it also sinks if you raise the water in a block, making realistic locks unworkable. I built a 4x4 water source, and the layer above I put 2 dispensers on two opposite sides with water buckets in them. When powered the water raises like expected, but the boat sinks all the way to the bottom. (I tried to make a larger lock, but that wasn't practical because of the way water creates permanent water sources. I'd expect the boat to float so you could do simple locks without crazy pistons.