There's no doubt that H&S legislation has saved many lives over the years, often by making employers stop ludicrously dangerous working practices or making them take sensible safety precautions -- the clue being the word sensible.
The point is that if somebody does something required by their employer as part of their job, it's up to the employer to make sure that all reasonable (not unreasonable!) safety precautions are taken, because the employee is being made to do the task by their employer and they owe them a duty of care. This includes climbing ladders, or pylons, or church spires.
If somebody climbs a ladder of their own volition at home and falls off and breaks their neck it was their own choice and they take responsibility for it -- which doesn't mean the ladder manufacturer shouldn't attach safety precautions to it, but if the user ignores them it's then their own fault. However sniggering at them probably isn't the right thing to do, given the number of people who are killed every year falling from ladders (maybe a hundred or so in the UK?) it's actually a sensible precaution, even if it doesn't look very macho ;-)
If a hundred people died every year falling into locks -- for example, more than died falling into rivers and the sea -- then it would obviously be sensible to put up warning signs at every lock, and possibly even fences to keep the public out at the worst ones (it works for trains...) if this was possible.
But this isn't the case, in spite of the fact there are thousands of canal locks many fewer people drown in them (less than one per year?) than in rivers, lakes, and the sea -- which can't all have warning signs fitted. All have the common factor that they're full of water which people can't breathe, as any four-year old could probably tell you.
The situation is different with CART workers, they are working on the canals as a job and so it's up to CART to provide them with safety equipment (lifejackets, harnesses etc) and make sure they use them. For people hiring boats it's the job of the boatyard to provide lifejackets and tell them about the risks with locks, then it's up to the hirer whether they follow this advice or not. For private boaters and gongoozlers (including kids who swim in locks) what they get up to in or near water which is self-evidently capable of drowning somebody is entirely their own responsibility.
The exception to any of this is if there are unusual hidden hazards which the man on the Clapham omnibus might reasonably be unaware of -- for example a dangerous weir. In this case warning signs (or barriers) are appropriate, and CART erect them because if they didn't they could not unreasonably be held responsible if an accident happened.
But prominent (otherwise pointless...) notices at every lock saying "Danger, deep water, risk of drowning" -- what next? Similar notices at every pedestrian road crossing? Should every knife have an embossed (so it can't be lost) warning saying "Danger, sharp knife"? How about warnings on packets of crisps saying "Danger, crisp crumbs may cause choking"?
These might sound ridiculous, but any one of them would save more lives than notices at locks...