I was once told that locking the prop in reverse was the most efficient but from my experience (and I've also seen a video demo) I am 100% certain that drag is minimised by freewheeling the prop in neutral. On most boats I've sailed recently, speed seems to drop by about half a knot
in lighter airs when reverse is selected.
Of course there are downsides in freewheeling. Firstly there will be more wear on bearings in the transmission
system, plus cutlass etc., often there is a noise
problem in aft cabins and some gearboxes rely on the engine
running to lubricate them, which won't happen in neutral.
So what do I do? I am prepared to accept a little less speed in lighter airs, so I lock the gear
in reverse as the potential damage and consequent cost of freewheeling could be high.