I don't use the gym at all now, but before covid I was only using the gym to swim laps and did the rest of my exercising at home. These are the annoying things that I see people do in the pool:
Some people slap their arms and legs on the surface of the water constantly while they swim. I think this is the equivalent of people yelling and grunting loudly while lifting weights. All the splashing is wasted energy and doesn't make you move any faster, and you don't get the benefit of the resistance of the water when your limbs are outside of it. The worst part is when you swim past these people and catch their splashes or wake while trying to take a breath. But it also distracts from the state of mind you need to achieve in order to swim laps for an extended period of time.
People who don't know how to lane share. If you're serious about swimming then it really sucks to share a lane, but especially if its with someone who doesn't swim close to the same pace as you, faster or slower. Usually the faster swimmers are experienced so they can spot who is a comparable swimmer, but the slower swimmers either can't tell or don't care. Those people usually spend most of their time resting at the end of the lane, but when they do swim they don't respect that both people have to share the lane and drift across the mid point. Whenever I get interrupted part way through trying to improve my half mile time I know its going to be by an old man with an enormous gut, a discolored bath suit he bought on vacation 30 years ago, with full face goggles and a snorkel who wants to swim the length of the pool once over the course of 15 minutes and he's got to use my lane to do it.
People who tread water or do water aerobics in the lap lanes. Most pools have an area designated for that kind of activity, but for some reason people use the lanes anyways. This is especially bad when it forces people swimming laps to share lanes. The same goes for people who just finished working out and want to cool off in the pool. It's annoying, but those people pay for the gym and get to do that so it doesn't annoy me too much as long as they aren't too loud. But the big problem with those people is that I know what they didn't do, which is the next thing.
People who don't shower before getting in the pool. Its disgusting. It's always people who aren't serious about swimming, or who are just jumping in for a minute, and it ruins the pool for the people who want to use it regularly. It's bad enough when its someone who just came into the gym and hasn't showered since the morning, but people who are sweaty from working out and jump in the pool without showering are real assholes and shouldn't be allowed to use the pool if they do that.
Swim classes. For some reason every gym has swim lessons for kids that shuts down half of the pool and they tend to do this during the busiest hours of the day. People say "their membership is paid for too," but our memberships don't cost the same, and those people would throw a fit if half of the free weights, circuit machines, or whatever other equipment were reserved for a class every day during peak hours. I feel that way about all classes taught in a lap pool that are not specific to swimming laps. Swim team meets make sense (still really annoying), but the kayaking classes that used to happen each night at my gym were ludicrous.
People who sit by the pool and use their cell phones. I know nobody is doing it to get footage of me, but it still makes me self conscious. It's tough enough to work out in front of other people without being made to feel like someone might be recording you whether its because they like how you look or don't like how you look.