When you press the “Lap” button in Boat Timer the Lap button disappears for that boat so you know you have recorded a lap. The button reappears when you press Update on the grid to sort the race into the order of the last lap for the sailors.
To make it more obvious which boats have lapped and which have not, the latest version of Boat Timer (version 21160) also turns the boat’s box grey. So only the white boxes have yet to have a lap recorded. Just like the lap button, the normal colour is restored when you press the Update button to re-sort the grid.
As always, the update to the Boat Timer app is automatic – if you close and open the app a couple of times you should see the new version.