Ironman University Certified Coach, 16x Ironman Finisher, 8x Kona Finisher and author of "How To Swim Faster in 30 Days", Wendy Mader explains in detail, the 5 issues that can cause breathlessness in the water:

Not exhaling Immediately before you inhale
Dragging lower body
Kicking too much
Dropping your arm too soon when you rotate
Conditioning