I gather that a low scale value for water will lead to smaller islands, so a large one should yield big continents?
From that I'd expect that "coverage" determines how much land and sea I am going to get overall. Less coverage -> more land, with less sea in-between?

The map preview doesn't provide enough coverage to tell a difference between the settings, so a few explaining words would be nice.

The sliders should function the same as in vanilla mapgen - island start just changes the map's starting point, if that makes sense?

I can't change the sliders' tooltips since, well, that's vanilla functionality and altering that would change it even when using vanilla or other mods' generation presets, but I can maybe add some extra info to the "Information" tab here.