Core strength is used in any dynamic pose and is part of many static poses.
The choice of poses and sequences can also develop both upper body and lower body strength. Again both in a dynamic and static manner.
As long as proper movement patterns are followed it is functional strength that is being developed. A yoga program can be designed to use specific movement that is applicable to the sport of swimming.
The yoga for swimmers article below goes over three poses that you can use to strengthen your core.
When working with coaches on implementing yoga into their programming the development of the core is always one of their biggest priorities at any time of the season. A strong core is needed in all strokes. A strong core connection links the upper body to the lower body, creates propulsion in certain strokes and a well conditioned core reduces resistance.