When you launch into the pool, while underwater, tap A and B but not fast. There is a rhythm to maintain underwater speed that I had to get a feel for. Careful to not drown.

Then, after you come to the surface, you perform the stroke by tapping A. Tap A again the MOMENT the swimmer finishes her stroke. Try to minimize the gap to make it a continuous swim. If done correctly, your speed should greatly increase. For the butterfly, breast and freestyle stroke, try to come up with a system of coming up for air that works with the rhythm of strokes you're tapping out.