Didn't get a chance to work on things yesterday, so I tried to power through a bunch of smaller things I knew I wanted to do before I went to sleep.
I spent most of the time trying to get proper lap times as well as best lap time working correctly. After that, I spent a bit of time getting 'simulated' terrain working, as well as adding a small bobbing animation for when I eventually get a submarine sprite in there. I say 'simulated', as since I couldn't have 3D terrain working with my current setup, I'm modifying the values using a terrain class and that heatmap I made earlier. Most of that is shown in some way in the GIF below!
Here's a proper video of how the lap times work. It ended up being way easier than I expected, but I had to get around confusing myself with a bunch of values. This video's a bit older than the GIF so it doesn't have any of the terrain or bobbing stuff.