Here is one of our favorite core exercises. We call it 30-60-90 for each of the body angles where we pause.
Have the swimmer be seated on a physio ball and have a teammate stand on his feet. The swimmer should then streamline and hold a straight line from wrists to shoulders to hips as he moves through the different angles. We like to go for 1:00 straight shifting positions about every 3 seconds and finishing with 10 seconds holding the 30 degree position.