(I ask this because it removes exploit in my modpack, where all water infinite below sea level, but finite above).

Infinite water is in the nature of this mod, how would a config option help?

i use Water Control Extreme mod, which makes water finite.
i set it to finite water only above sea level, and infinite only in wet biomes below sea level

with pretty beaches - i check - in survival when i dig on any levels in any biomes - it floods.
if it possible - I would like it to be only below setted Y level. in my settings - it sea level.
better, if in biome whitelist)

There already is an only-at-sealevel check which is on by default. Make sure you're on the latest version.