I let my PC compile for the last several hours. All countries but... seem allright:
1. Australia-Oceania from Geofabrik simply misses a lot of sea. No apparent flooding though. Also the overview map has undefined holes, which should not happen IMHO.
2. Well no flooding but... in Germany. I do think that the splitter has somehow created the problem. I have uploaded the broken tile for analysis here: http://openmtbmap.x-nation.de/maps/beta/63710031.osm.gz
3. And I have no idea at all why this is happening. When I zoom into Athen, 700m or 200m (medium detail) crashes Mapsource. (6.13.6 as well as 6.15.7). Not sure what is happening here.

Depending on the sea complexity some countries really take a lot more time for rendering now. (Norway went from 5 minutes to 45 minutes). I don't really mind as my upload speed is still a lot slower than my PC compiling on my weekly updates.

I have not yet tested the patch for closing the sea, will do so. I noticed