@Kaari
Yes, it's easy to add tile sources. I've just made a try for OpenSeaMap, and it's basically working. Except that OpenSeaMap is an overlay tile source to be displayed on top of another tile source. Currently Mæp can only display one tile layer at a time, so now one can only see the sea marks but no coast line or so ever !

I put this in my TODO list to have the possibility to display a tile layer on top of a tiled map. I cannot guarantee when it will be available though. But the implementation seems not too complicated from the current code base.