Swim diapers do not absorb any urine. If the swim diapers absorbed the urine the swim diapers would absorb the pool water which they're designed not too.
Swim diapers are only meant to hold poop so you should only put a swim diaper on your toddler or baby just before getting into the swimming pool because if they wear the swim diaper before that such as the car ride or walk to the swimming pool then the urine will leak all over if they pee in the swim diaper.
Have your child wear a regular disposable diaper that will absorb urine on the ride to the swimming pool and then put them in the swim diaper when they get ready to go into the pool.