For our first climb at montagu we arrived at the crag only to find police divers fishing a fresh body out of the lake/pond at its base. Didn't exactly put us at ease with the whole security thing, but the guy who runs the hostel at Boven was really good and even sent someone along to keep an eye on us when we were climbing at some of the dodgier crags.
Don't let it put you off though, there's great climbing in SA. The sport climbing in Boven is indeed awesome (and extensive), especially the routes around the waterfall.
Edit: I should point out that the body in the pond was NOT a climber.