The racing boats initially have a Lap button on their boxes. When it is time to finish the race, you need to change the “Finish Mode” switch (on each black start box) to change the Lap buttons into Finish buttons.
Press the Finish button to record a finish for each boat. The Finish button disappears when you press it and the box changes to light grey so you know that boat has finished (it is also grey if the boat is disqualified or retires from the race). A tick and a finish time is displayed in the box of each boat that finishes.
If you make a mistake, click on the boat’s box once to select it (it turns orange) and you will see an “Unfinish” button on the black bar below the grid. This sets the boat back to racing.