Wouldn't be better just don't allow to town to build on sea level? I don't know how it affects the growing of town, but for me it doesn't feel like the right approach to disable the whole terraforming because it can occurs 3 times in the case of 12k towns.

I believe this is probably the wrong fix. A town that has elements at sea level can still flood itself by lowering terrain into the ocean outside of world-generation, so it doesn't fully fix the reported issue, while also being massive overkill.

Prevent terraforming of tiles only at sea level, but regardless of worldgen?

A few observations:

• The coded piece of code is only used for "natural" road-layout generator of towns. Not for the 2x2 and 3x3 grid layout. So it is a bit deceiving to use it as "argument" for this PR
• After town generation, towns can still flood their own town
• If someone has build_on_slopes set to false, his towns out of map generation will be horrible. This because of this line of code:

if (!_settings_game.construction.build_on_slopes || Chance16(1, 6)) LevelTownLand(tile);

All in all, you managed to fix your use-case, but disregarded any other use-case out there ;) I am pretty sure this PR makes things worse, not better. To put it in numbers: it made 12k out of 12k towns look worse, while fixing 3 out of 12k towns. Pretty sure that is the wrong balance ;)

I think the suggestions given in this PR are solid: prevent towns from terraforming below sea-level sounds like a more robust and "this doesn't brake most cases" kind of solution :)

I am going to close this PR now, just to close this chapter. Please feel free to create a new PR with a more robust solution; and please take the time to test other use-cases :) Tnx!