But this 16ft dsmb has an open bottom just like the shorter ones and it seems like it's meant to be deployed in water from depth.
What you are saying isn't correct, it is meant for surface deployment in high seas or in rescue situation. Open bottom just to give you one other option to inflate the tube with your second stage on the surface just in case you are having troubles doing it with orally.
Use the right size tool for the right job. Big size isn't the right size here.