There are times (mainly whilst out at sea in very low density traffic) where being completely out of your box will have negligible impact on your ability to helm your boat.
However if you are for instance trying to tow waterskiers, wake boarders, Ringo's, etc... in a busy designated ski area where not only do you need to keep an eye out for other boats but also fallen skiers etc in the water and you are constantly having to think 3 moves ahead simply to avoid killing someone. Drinking and Helming in this type of situation is plain and simply reckless.
As to the question do we need laws to prevent the worst happening - its like everything - a few irresponsible people need to be controlled so everyone has to suffer an inconvenient law.
With reference to this accident - both boats were acting irresponsibly on many levels - drinking being just one of them. It is a bloody long and expensive taxi ride from falmouth to st mawes by road - but just a few minutes by boat - if drinking and helming had been illegal I bet these people would not have even been on opposite sides of the river estuary for a night out.
There is even a fairly in expensive water taxi service aquacab
that operates in the area that they might have taken if there had been a drinking law.