A lot of swimmers flop-over backwards on their entry. This is due to their core being too soft or not strong enough to hold position.
On the entry a swimmer hits a resistance that is 800x thicker than air. While the top of the body is slowing down abruptly the back-end keeps going. This often causes a backwards bend and slows a swimmer down dramatically.
Core exercises will help improve this problem in addition to the awareness that it is happening.
Besides Pilates, one of my favourite exercises is using a hula hoop. Swimmers enjoy this exercise and so it is much easier to make sure core work is done.
At a coaches clinic about 20 years ago (ish) I listened intently to Michael Phelps’ coach after Syndey. I noticed that one of the interesting things he did not do was use a hula hoop. So I waited my turn to chat with him after his talk. He was very appreciative of the idea and said he would incorporate it right away. I hereby take all credit for Phelps’ success.