When you try to place a boat in water, if there is a block in that water, for example a slab or a rail at the bottom, the boat will sink and be unable to move instead of floating on top. If you are riding the boat and then cross over a block like this the boat will still sink but you will be able to move around like normal. It makes no sence that you should be unable to ride a boat somewhere just because there is a rail or carpet underneath you, nowhere near touching your boat.