>Seems to me the only effective and enforceable redundancy that can be easily be imposed by regulation would be mandatory tug boats.

Way it worked in Sydney harbour 20+ years ago when I briefly worked on the wharves/tugs, was that the big ships had to have both local tugs, and a local pilot who would come aboard and run the ship. Which seemed to me to be quite an expensive operation but I honestly cant recall any big nautical disasters in the habour so I guess it works.