Since you are a non-swimmer, the side stroke with a scissor kick works better for you, as your running history has developed your hips more in the scissor-kick plane than in the breaststroke plane.

The front crawl is characterized by an alternating overhand motion of the arms and a flutter or scissor kick. Athletes use their ... swimmer's head must come above water during each cycle of a stroke ...

That said, not all ab exercises are created equal, and one that doesn’t get a lot of attention (but does a great job at working your core) is the humble scissor kick ... by the side of your ...