I tried to use the 'depress' terraforming tool to tweak a shoreline. But all the surrounding water turned to land, at the same level as the water was. Trying to fix it by lowering the edges causes the dry area to expand. If I lower it enough, I can fill the basin with water using the terraforming tool -- but it's still surrounded on all sides by land.
So it looks like that if I use a "depress" or "increase" tool on a shoreline, the only way to fix it is to depress every single connected water tile, then fill the resulting basin with water.
(I even tried moving my land-cancer towards deep water, to see if deep water broke the turn-to-land thing, but deep water turns to land at the same level of the water surface as well).
Is there a way that I can fix this (turn everything that used to be water before it was lowered, back into water) without going back to an older save? Also, is there a way I can terraform near shorelines without causing all the adjacent water to dry up?
I tried to find an existing post on this issue, but my search-fu failed me.