I was diving in BC last year and came across a small GPO (couldn't have possibly weighed 10 pounds) on a drift dive. I allowed it to touch my glove, which it grasped, gently at first and then firmly.
I was drifting in a 2 knot current and let go of the portion of the rock wall I was holding, and this little octo was able to hold me in place with what appeared to be no great effort on its part.
I have been climbed by a larger GPO before, but never pulled into a den (actually I think that would be a pretty cool story to tell). But I feel certain that a good sized GPO would pretty much have its way with me underwater, at least in a wrestling match.
So, IMO, GPOs are what I would consider very strong in an underwater environment. Of course, such encounters where I am actually touched/grabbed are very rare and I consider them an extraordinary treat... so given how reclusive a GPO is, I can't imagine it intentionally grabbing for a person at the surface.
Not that any of this has anything to do with a Colossal Squid.
I have read some strange (and some dubious) reports of intentional encounters with the Humbolt squid by divers in the Baja area. If the reports of those critters' behavior are anywhere near accurate, they are frightening indeed.
I have been in the water with a pod of 20 orcas, which I would consider the apex predator of the known ocean, and never felt the least bit scared. I hope to dive with great whites sometime as well. But if the reports are true, I wouldn't want to be anywhere near the humboldt squid. I am guessing, based on the "weapons" and the reports I have read here, that the Colossal squid would be an order of magnitude more terrifying.
-david