Just LOOK how much vocab you can teach when you take your little one swimming. And this is MINIMUM - there'll be so many more things specific to your local pool that you can talk about.
LOOK AT EVERY OUTING AS A LANGUAGE DEVELOPMENT OPPORTUNITY. If you can say all of these words when you go to the pool - you're winning. Remember - children only learn how to use words by hearing them multiple times in context. 'In context' just means in the moment, in the environment they're supposed to be used in. For example you could say the word "locker" 100 times to a child at home and they're unlikely to understand it or use it, but if you say it a few times in context (standing next to the lockers at the pool) then they'll get the gist!