Interesting thread. Three thoughts cross my mind.

1: Sigma's OS might not like the monopod. Early generation IS/VR lenses from Canon and Nikon hated tripods. Later models with second and subsequent generation IS/VR auto sense and adapt themselves to hand-held or tripod.

2: The general rule with moving subjects that don't move too fast is keep the IS on. (I've never done surfing, but I would certainly use IS if I did.)

3: The AF system on the 5D II is seriously primitive - the worst AF of any recently produced camera I know of. I only ever use mine for landscapes, and even for that it is poor. So you have to work with the damn thing and force it to do what you want! But you are a 5D II owner, you know this.