I don't know. It would depend on the depth, but not even that necessarily. Now those pipes are no thin drinking straw, but think of what a bundle of fragmenting hand grenades could do. Packed watertight, with ballast, if need be. Downed from some trawler, yacht, whatever with good sonar to see where exactly, combined with with some contraption, maybe mechanical timer and CO2-cartridge blowing up a balloon which is pulling their pins. Or something more professional, which some rogue Spetsnaz surely would have access to?

Same delivery method, no divers needed. Could be almost anyone.

(shrug)