I tried both Zef and Waterspeed with my Apple Watch and they were very similar. I settled with Waterspeed. There's a ton of features I would like to see (time on foil, VMG, auto-pause, better replay, map indicator for top speed, jibe count, tack count just to name a few). That being said, it has helped me track my sessions with a bit of stats. Not 100% sure about the speed accuracy but that's not solely the responsibility of the app as the hardware plays a big part.

Windsport Tracker for android phones is great. Calls out speeds and last run max as you go. Waterproof pouch and tuck it in your wettie