App recommendations

Sorry nobody has replied to you yet on this, I was going to say something but I know next to nothing about iPhones. I do know that the GPS in them is not good enough for accurate speed calculations. It's fine if you are using it to navigate to somewhere in a car or something, but it's just not designed for the sort of accuracy needed for this kind of sport.

Having said that, there is nothing wrong with giving it a go to see what sort of speeds you can get from an experimentation point of view.

I suppose, ideally you'd want an app that allows you to export your data files in some form (gpx or fit files most likely) and that would allow you to use one of many freely available tools to do further analysis, but I expect you'll be disappointed by the results in short order.

The easiest way to get started without astronomical cost and without the risk of drowning your iPhone (waterproof pouches can and do fail when crashing at speed), would be to get a decent GPS watch like the Garmin Forerunner 255, which has an "approved" GPS chipset for the kind of accuracy needed. There are some few other models that also work from Garmin, but the 255 is usually easy to find on sale and probably one of the least expensive entry points. Plenty of hobby/beginner GPS sailors in our neck of the woods use this watch, and it is also approved for use in various competitive settings too, so it's a solid investment in your future endeavours.

There are other topics on this forum regarding device recommendations. Stick around and you'll learn a lot.

I've used Strava in the past, starting the "windsurfing" activity before heading out. As Dylan says, phones are not very accurate for speed measurements, so any app is going to be limited by the hardware's capability. In the case of the iPhones, location is logged at 1 second intervals, and speed is calculated from distance traveled in that one second. For mapping out a track and overall distance, it's good enough. If you go in the water and you have the phone in a waterproof bag in your vest or something, the GPS reception basically cuts down to near-zero as the signal attenuation is very high underwater, and it'll often give a big velocity spike as it tries to resolve it's updated location when you get out of the water.

Select sport watches use doppler-shift based velocity measurements in addition to logging location, and often do so at a higher speed (I think some are up to 5 times per second, but can't confirm). The key thing though is that doppler speed measurement. I got a Coros Apex 2 last summer, and it's been awesome. I still bring my phone with me in a water proof pouch for emergencies, but don't bother with Strava anymore.