Posted by: rjd9999 on Nov-18-12 3:45 PM (EST)
With the suction cups/straps. Better, most likely, than any attempt to wedge an amorphous bag of water into a secure location or having it lie between legs.
As for rolling, I still don't think that water in a container (as long as it doesn't interfere with mobility) will have an affect on one's ability to roll. We're talking about 80 lbs. of water (about 37 kilos) and I've rolled boats with considerably more dead weight in them.
The way I see it, the container will either shift or not during the roll. If it shifts, it will simply fall to the lowest point of the boat during the roll, which may actually minimally facilitate rolling. If it doesn't, it would be no different than any other 80 lbs. of dead weight in the boat.
But to answer the question as to whether I've done this with a water bag sloshing under my legs, no, I haven't.