AP Valves Divers Closed SMB is the self-sealing version of AP Valves Buddy SMB, which has been a mainstay of any divers' kit since early 90's.
At first look this SMB -affectionately referred to by most divers as a "safety sausage"- looks like an open ended from the bottom but as soon as you fill it with air, the internal baffle is foced shut, which prevents it from deflating on the surface. Very clever feature indeed!
You don't need to worry about the sausage bursting either as the over-pressure / dump valve on the SMB effectively prevents this from happening.
These SMB's come in either all-yellow or all-range high visibility colors, making them easy to spot from the boats. They also roll up to a very tight, neat package, so you can carry them with without adding drag or too much weight to your gear.