I agree with Dan -- bumping into a submerged tree in blackwater is a jarring experience. It always resulted in a jolt of adrenaline, though the trees never did any more than startle me. Blackwater diving requires a certain nervelessness. Panic is the enemy.
Sorry thats not me in the video that was for Asparagus. That is a diver out of CT Matt Pegolo. The video just gives you a good idea of what its like diving fresh water rivers in the Northeast. The problem in the Charles is it very bad viz in warmer months.