Okay, I realized my map was getting bizarre because two modifications played together: I had modified the map to get the number of tiles of the EARTH map, but I had forgotten I had also used the trick explained in this thread (I had changed the value from 64 (40 00) to 12345 (39 30).
This combination was interesting though, because it provided much less random jungles and swamps. Problem is, I got one enourmous continent with big inland seas, and a much smaller one (someting like 80/20, with a small isolated island), while I'd like something like 50/50, 60/40, maybe 70/30.
So I decided to put the values back to 40 00. And then...
I get enormous inland seas which almost make the game unworkable, or at least very unpleasant to play ("Yeah, 1st city near sea...Fudge, it'a lake and big useless one at that !").
I don't know how algorithms work. What I need is simply an intermediary value between 64 and 12345 which allow two or three big continents while almost excluding inland seas (Inland seas in the vein of the EARTH map will do the trick). My ideal is still a EARTH-like map without it being a copy of it.