I like stability training of the core for swimmers. Think plank and bridge progressions. You want the core to be stable where there isn’t a lot of twisting side to side. You want the stroke to be efficient regardless of the stroke being swum. This is specific to your question of what core work could be done to enhance swim training.
You also want to strengthen the back, chest and shoulders in all ranges of motion. I like tubing for this and often will use tubing with additional weight from dumbells. It more closely mimics the push and pull of the water. I also try to have clients, as well as myself, keep their hands lose when gripping the weights when strength training for a swim as to mimic hand formation in the pool.
Of course, nothing compensates for time in the pool, but core and strength training can help make a swimmer more efficient.