Try pyramid sets when swimming. Breathe every two strokes for one length, then every three, then four, and so on until you’re doing a whole length without breathing. Then set off back down. The final length breathing every two will feel brutal! One pyramid will probably be enough – don’t rest between lengths, only at the end of the pyramid.
I swim most lunchtimes (lucky to to be near a club with an outdoor pool) but I’m not sure i’d do the non-breathing bit, still maybe it’s a technique i’m not familiar with.