Hanging Windshield Wipers. Advanced core move relatively speaking! If your pool deck has a pull-up bar on it, this is one to impress your teammates with! Scaled down options include a smaller radius of movement or hanging knee raises. If for some reason they're too easy, put some ankle weights on!
Push-Up Plus Body Saw (Foam Roller) - connectivity between core stability and shoulder control. Typically a good warm-up or "pre-hab" exercise. Harder than it looks!
Floor Seated Single Arm DB Arnold Press. Don't just focus on dat pump, lock in dat posture prior to pressing overhead!
Hollow Hold aka Banana - basic but effective. If your whole body shakes while you hold the position, you're doing it right! Do your best to pin your lower back flush against the ground.
Floor Seated Bottom Up Kettlebell Press - bottom up kettlebells are a great way to increase the challenge of movement without adding load. This is good for swimmers out there with injured/tender shoulders.
Core involved in a wicked shoulder bolstering move!
Palms Up Banded Pull Aparts - warm-up or good for general healthy shoulder movement!