Swimming is crazy hard. It takes a ton more of energy to swim from point A to point B than it does to run the same path. The guys who get really good at swimming need to make sure they do a lot of volume in the pool, and burn thousands of calories as a result. I have a lot of respect for pool-dwellers.
Heavy (1-3, maybe 4 reps at a time) weightlifting has a place in all sport-specific training programs, and swimming is no exception. Maximally contracting the muscles encourages the development of type IIB (fast-twitch) muscle proteins, and teaches the central nervous system to fire with maximum intensity. Type IIB muscle fiber development is crucial for any sport you can think of.
In swimming, the prime movements are around the shoulder, namely shoulder adduction and shoulder extension. These movements are controlled mainly by the latissimus dorsi, aka the lats. Flying-fox lats are a common sight on top swimmers for this reason.
A lift that fits this bill is the weighted overhand pull-up. These use the lats and upper back a little more than underhand chin-ups. It has the same shoulder movements, and it’s my belief that the more weight you can shift in this exercise (your bodyweight plus any added plates you add using a dip belt) compared to your bodyweight, your swimming performance will improve.
If you’re a swimmer, try this lift out for two to three months, and see if it has an effect on your swimming. I’ll bet the ratio of your WOPU to your bodyweight will become something of a performance indicator.