Are you using the no-mp option? This fills every tile with a sea polygon, then overlays a land polygon on top where it finds natural=coastline. Thus even tiles with no land on should contain a sea polygon.

You could try manually adjusting the split tiles, so that each one contains some land. You would need to edit splitter’s areas.list file to do this (it’s a painful process, as you need to convert lat/long into Garmin coordinate system).

To support sea generation with the no-mp option, you need a couple of custom definitions in your polygons style file and your TYP file.

natural=land [0x010100 resolution 12]
natural=sea [0x32 resolution 12]

and have appropriate polygons defined in your TYP file to match these. You may also need this if you use the mp option but I can’t remember.

PS As a test, try creating the map using my style files, mkgmap option file and TYP file and see if this solves the problem or not.