The phrase "poor swimmers" means people who are not very good at swimming or struggle to swim.
Sentences with «poor swimmers»
Most robotic fish are like fish out of water: They're surprisingly poor swimmers, and they often scare away any other creatures they come in contact with. (sciencemag.org)
Very young and very old pets are generally poor swimmers as well. (thedrakecenter.com)
If your dogs are like mine, they don't really like the pool and are very poor swimmers. (bricktownvet.com)