Boats will not rise above the level of  the water on which they were placed. If water is placed on a hill, and then the dirt of the hill underneath the water source blocks is removed, down to one block below the water surface, the boat can traverse the channel with the player as long as it does not sink one block. You are then under water, submerged, riding the boat at full speed through the channel and able to work with tools and have infinite breath while working at this level.

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