Swimming is very different to other types of sports training. When you're in the water, you do not have the ground to push off to help you generate momentum and help you move faster.
This is where your core, legs, and arms come in. When swimming you use a mixture of your core to keep you afloat and your legs and arms to push you through the water.
However, having these muscles already strengthened before hitting the water can help to prevent incorrect movements and can also contribute to preventing muscle fatigue.
Some great exercises for swimming include:
Medicine ball push ups - help to strengthen the chest, shoulders, and triceps. It also actively engages your core helping you to improve stability and movement.
Kettlebell Swings - The kettlebell swing is great for building strength in your core, glutes, and hamstrings and enhances power in your legs. It also helps to improve your cardio vascular muscles which in turn will assist you in the pool.
Plank Row - Yes, everyone hates doing planks. But the plank is the best exercise for strengthening your core and your back. This helps to create stability in the pool as your core can hold more of your body weight without tiring quickly.
Rotating one arm row - with this exercise you are building the muscles around your scapula. You'll find once you strengthen this muscle you can get a better range of motion which will help with those long strokes in the pool.