You can strength train a bunch of stuff and hopefully maybe improve some of the muscles needed in some of the ways you use them, or you can strength train the exact muscles you need in the exact way you use them while swimming:


Find a cable pull machine and set the cables and handle to pull from as high up as possible.
Grab a cable and start backing way up and then get on your knees and straighten out your arm, so your hand is stretched out flat, arm and back are flat, hand out in front of you like you are about to engage a stroke pull.
With your head down and back flat like you are swimming at where the cable is coming out of the machine, pull the cable back with a high elbow, exactly like a good swim pull stroke.
You should use enough weight only be able to repeat the pull about 12 times. If you can only do 10, drop some weight. If you can do 15, add some weight.
Repeat on the other arm.
Repeat both arms for 3 sets the first time you ever do it, wait a couple days, do 4 sets then on out.

For those thinking that doesn't do core, try it and watch what happens to your midsection when you try to pull and then push a cable from far above your head to past your waist on one side of your body, then the other.