Resistance training is certainly going to go a long ways in swimming, but let me just point out a few areas to concentrate on.
The shoulders and rotator cuff muscles – These are small muscles that fatigue quickly as they’re not used much in daily life compared to swimming, so it’s very important to train them in both strength and endurance. Any direction your shoulder moves, you should train that movement.
Glutes and hamstrings – In my experience, most of the population is strong in the quadriceps but lacks the balance in the posterior chain, that is the back side. You would do well to perform hip extension exercises, and total body lifts like deadlifts and squats, though you should consult a professional if you are not one yourself as poor technique can quickly lead to injury.
Chest and back – Triceps and biceps are not as important as they are the prime movers of the elbow. Swimmers’ movements occur primarily at the shoulder. The upper and middle back and often very neglected areas of the body.
Core stability – Anti-rotation and dynamic stability exercises will strength the core and prevent energy leaks, making each stroke more efficient. Also note, consider the whole “core” not simply just the abdominals and obliques.