I found huge improvements from taking on gyming while swimming - specifically muscular strength and endurance. That is, until I overtrained and heavily damaged my back. Be wise and know your limits!
I think two to three swim sessions to one gym session is a good ratio. If you're a sprinter, maybe more towards two to one swim sessions to one gym session.
How many sessions a week that ends up being depends on how into the sport you are! A competitive swimmer may be doing 7-10 swim sessions a week, coupled with 3-4 gym sessions would be a nice mix.
Make sure you give enough time for your muscles to recover. Throwing gym into the mix puts a much higher demand on your muscles.
Sets and Reps
I did freestyle mid-distance events, 200 m up to 800 m, so my focus was more on endurance reps.
For standard workouts, I would do 3 sets of 20, and gradually increased weights accordingly. I did them at a similar speed to swimming stroke rate, except for legs of course.
I frequently did circuit training - 30 seconds on, 30 seconds off, go to the next exercise, around 70% of my max or greater. This was the money, I loved these workouts!
One stupid set I frequently did was pyramid sets on quads until muscle strain. 20 reps, climb weight until you injure your muscles, then climb back down. I actually didn't realise that I was straining my muscles, the 5+ days for my quads to recover despite my well conditioned muscles should have been an obvious clue!
Think through what muscles hold you back the most, and focus on them.
- Cable flys - these were my favourite! Go for pulling from
full extension, this is great for having a powerful pull at maximum
reach. Slight bend in the elbow, just like your initial underwater pull.
- Biceps curls - pretty boring, but important
- Triceps - use free weights, extend from half to full. Heaps of ways to do this and they're all pretty similar.
- Shoulders - free weights, extend to horizontal on your side and front
- Rows - your traps are helpful in the pull sequence, but more so it's just good to work the opposite muscles so you keep your muscle development equal.
- Bench press - covers the above muscles quite well.
- Pull ups/machine pull downs - real important
- Abs with weights - whether it be sit ups with plates or pulling on a cable, it's all good - gives your off the wall dolphin kicks more grunt
- Back extensions with weights - just as important as the abs
I know some people like to throw squats in the mix for more off the wall push power, but I'd say that it's lesser importance unless you're in some serious competitions. In which case, you would have asked this question to your team physio/gym trainer!