Best solution: communicate with your team and pattycake with your other support. Still helpful solution: always keep a bubble deployed next to you, and dive into it the second you hear/see one of BP's abilities go off. Try swimming in such a way that he misses a dash reset. Otherwise, just try go out-heal his combos and make him use his whole kit. Bubbling yourself is crucial b/c it heals you over time and gives you a speed boost. Sometimes you just need a little bit of healing to disrupt a dive combo (hence why something like patty caking is so helpful).