Cuttyhunk Island has a pvc tube extention permanently mounted to the top of the Town mooring balls. The looped end of the mooring line slides freely through this tube to the chain swivel, several feet below the surface.
There are pluses and minuses to this setup. The loop is kept out of the water, therefore not covered in slime - easier on your hands and deck lines. It's also fairly easy to slip a line though with lower freeboard vessels in relatively calm conditions.
The problem with Cuttyhunk, is the predominately high winds - seemingly always piping up when we come in for a mooring. During these higher wind conditions and with our high bow freeboard, it's tough for the crew to snag it with a boat hook and pull the line up high enough to thread a line through.
During gale winds last year we snapped a hook in two and lost another overboard on another occasion.