Apple currently does not allow developers control over when the Apple Watch screen turns on or off. This means that MySwimPro cannot automatically illuminate the screen.
What we recommend as a compromise that keeps the water lock on is to simply hit the side button (not the crown) when you want to turn the screen on. It will illuminate the screen, not obstruct it with the water lock popup, and is more responsive than the wet touchscreen.
There is a setting to keep the Apple Watch screen on longer: you can find it in the Watch app on your iPhone. Unfortunately, this setting is not respected while water lock is on. So, a less-preferred workaround is to set your "screen on" setting to 70 seconds and leave water lock off during your workout.