Two great exercises that I often recommend to swimmers are the Quadruped and the Dead bug.
Both exercises have the emphasis on utilizing opposing limbs i.e. right arm/left leg and left arm/right leg.
1. While maintaining a neutral spine, kneel on the floor with your knees under your hips and your hands under your shoulders.
2. Raise your opposite arm and leg straight out with your abs engaged.
Perform a static hold on each side for 20 to 30 seconds based on your fitness level. Repeat 3x.
The Dead Bug
1. Lie supine with your lower back flat on the floor. (Note, you shouldn’t be able to fit your hand in between the floor and your lower back.) Hold this position throughout the entire exercise.
2. Slowly lower the right arm and the left leg down to the floor simultaneously. Keep the back flat and your feet off the floor.
Aim for 3 sets of 20 repetitions.
Give these two exercises a try and see how they improve your strength in and out of the pool.