WanMil,
Good news - I don't think it's your problem. I disabled the sea generation completely and the lines on the tile edges are still there.
Oops, I may have spoken too soon. With your v3 patch in place, I made a new map with generate-sea=polygons and not only are the lines still there, all my sea has disappeared! Don't worry, I now think the lines between the tiles are always there on my Baltic map and I just haven't seen them before because when you use generate-sea=polygons you get polygons covering all of the map. But when you use generate-sea=multipolygon, there are no land polygons and so the lines show through from underneath. Now why they are there, I don't know at the moment (I don't think it's the fault of the MP code). However, there is still a problem in that with your v3 patch in place, I no longer get any sea polygons visible when I use generate-sea=polygons. I wonder if the change you made to the size of the sea polygons is responsible for that? Could you please change that code so that it only produces the over-large sea background when generateSeaUsingMP is true? Thanks, Mark