If your boat so much as touches any rock that hasn't been mined yet, that rock attaches itself to the boat and you can take it with you. You can still pick up the boat, however.
The stalagmites in the water block passage when approached head on, but if you turn boat 90 degrees, you can pass through moving laterally. It seems only the bow of the boat has collision detection.
The stalagmite make travelling by boat impossible without the ability to move over them in that way, however. It would be nice if we could chop them down with the pickaxe to clear a way, or make the footprint of the boat smaller.
