Swimming is about 80% technique, 20% strength (go ahead, argue the percentages, but its clearly a lot more technique than strength). It is true you can get most of the muscles you need from just repetitive swimming - however as we age (over 1
the body's joints wear down faster (and don't heal as well as) than they used to under high-rep activities, and you're far more likely to suffer an shoulder or knee injury long before you can fully develop the strength that you'd have gotten in your teens.
For exercises: there are "generic strength" exercises and "individual muscle" strengthening.
"Generic" pretty much has anything to do with your core (abs/torso) The ones that benefit swimming most are static positions (like doing the plank), roman chair back extensions, or just holding your legs elevated above the floor. Yoga pretty much offers the best mix of static strength and flexibility for your torso.
Individual muscle exercises should focus on the back (pullovers are great), tricep extension, and forearms (wrist curls). For legs you'll get more from getting better flexibility with the muscles you already have rather than trying to build more muscle. Flexibility is also critical in the shoulders.