For people who take care of sites by canals, this may be worth trying ...
Last night I tried out using my watering can on a rope and dipping it into the canal. It worked really well and fit well in a bag to take home on the underground.
For people who walk or cycle past their sites, I've gathered up several of the cleaned out Ecover squirty bottles I've been keeping at home for people to keep in their rucksacks. They're strong and supple and seal well and can easily be kept stashed in a bag for unexpected trips past sites near water.
Maybe those collapsible camping buckets can also be put on a rope.