If you use newer iPhones, the video will shudder due to the built in electronic image stabilization of the phone’s camera. The electronic image stabilization tries to compensate / correct the slight, but ever present engine vibration, which results in a wavy video. I had this issue with my iPhone 6 and contacted Mads (owner, inventor, and maker of Ski Doc). He advised me to use a different (simpler) camera / older iPhone or to use the display/selfie side of the phone. That camera is obviously not as sophisticated as the phone’s primary camera.
I had a retired iPhone 5c lying around and now use that without any issues. The videos can be easily transferred between the devices using Apple’s built in Airdrop function.