Take a GPS device with you whether it's a watch, dedicated GPS like the Velocitek, an action-cam with in built GPS, or your phone running a GPS app. The official app of Joyrider TV is Waterspeed (but you can use others)
When you've finished your session, check the GPS and send the details of your top speed to totaljoyrider@icloud.com

Along with some info:
Helm + Crew name
Boat Type, location and evidence - like a screen shot of the GPS

Include a photo or video clip for the next update video if you like.