You can already wade through water and the increased fluid friction (aka air resistance) of water affects the physics based animation quite realistically. Currently water is not disturbed by interaction with other objects and I'd say that and buoyancy probably the most important next features for water.
Swimming and even basic boat travel don't strike me as particularly difficult to do but as always it's a matter of priorities and having a lot of stuff to do in just 18 months. Assuming the game is relatively successful we hope to continue improving it and adding features for some time after release.
I'm tempted to discuss inventory a little but that should probably happen in the Inventory handling