I'm trying to guess what they meant by dumping water, it could have been at Wat Kow Tahm. In the typical Thai bathroom we have water troughs, there's no shower. You actually scoop water and pour it on yourself to have a bath. So it's possible that this person was cleaning out those troughs and there's some mosquito larvae in there and some beings were dying in that way. So I'll consider this question with that in mind.
Intention is the key. Intention is the key to whether you are killing or not. You walk out on the path at night, and I'm sure it's happened to a few people here, you stepped on a slug or a snail and killed it. There was no intention to kill, the intention was to walk to the hall and listen to the Dhamma or sit in meditation, and so it was a good intention. It was an accident that the being got killed. That's one thing that could happen. Someone else may see an insect on the ground and they just go up and step on it. Just for fun, they kill it. That's different, that's an intention to kill.
We also look at it differently depending on whether it's an adult or a child doing it. The kid's usually doing it because the adults are doing it, too, they are just following what they see. They don't actually have a conscious awareness that this is a creature. It's just a bug, a nuisance. But adults, we could say, should know better even though they don't. So the intention of an adult killing a snail intentionally compared to a kid is very different Kamma, because the adult "should" know better.
However, just to live involves the process of killing.
A piece of paper, where did it come from? It originally came from trees, trees that were cut down. When it comes down it goes smash on the ground and all the bugs on the bottom are killed instantly and all the bugs on the top ,if they are underneath the bark, are usually killed instantly, too. A lot on the sides may fall off but hundreds of beings are dying every time a tree comes down. So in order to have paper we are in involved a process of killing. However the object of having this little piece of paper right here, the intention of having this paper is for Dhamma. So the intention behind having the paper was not to kill living beings, the intention was to use the paper for some good.
So the intention behind whatever we are doing is the important thing. If this person was cleaning the bathrooms at Wat Kow Tahm, their intention was good. Unfortunately the mosquito larvae can't always be saved and so some of them will die. The intention is the main thing though. The intention is not to kill, by accident some of the beings do die. We want to always look at our intention, we want to be pure in our intention as much as possible.
Now if we see that beings may die through an act of ours or through normal everyday living (just mow the yard, the intention is only to trim the yard but there's going to be a lot of beings dying in that), do a little Compassion/Lovingkindness wish for the beings that may die. There's bound to be some beings that die along the way but that's not our intention of why we are doing things. So we encourage very much when you know other living beings are going to die through your actions to do a Compassion/Lovingkindness wish for them.
However, make sure your intention is ok. Going back to the paper, if you have an intention to buy five weekly magazines, well you are contributing to a lot of beings dying. Do you need those five magazines? That may be something for you to look at a bit more seriously. Do you really need any of that? Looking at your clothes, how many clothes do you really need? The less that you want materially then the less you are actually causing other living beings to die. That's something else that you can look at.