Heres my 2 cents
Maybe instead of focusing just on water starts, the whole custom starting position system could be upgraded to allow you more freedom in your desired starting position. Perhaps you could select an initial altitude(ground surface included), maybe an initial airspeed, and initial heading. Would need some programming to ensure the aircraft is in an acceptable configuration for the starting conditions selected. IE, starting at the water surface doesn’t spawn you with the gear extended.
Also, if you choose a ground start, a dropdown menu showing the available aircraft configurations to start in would be cool as well. So you can select Apron, Cold and Dark, Taxi, etc. And that dropdown could be shown to let people choose for all ground starts, not just custom ones. With the recommended configuration for the chosen parking spot preselected by default. IE, choosing to start at a gate would select “Apron” by default but still allow you to change it, starting at the runway would choose “Runway” by default, etc. That way you don’t impede the flow of people who just wanna click through quickly, or require too much fine tuning for new users who might not know what all the starting states mean.
EDIT:
ALSO! This might be getting too advanced, but since the starting configurations are just .FLT files stored in the aircraft folder, you could also allow users to save an aircraft configuration and then start with that configuration from the menu.