How do I change blank OpenStreetMap tiles to be classified as water using PostGIS and GeoServer?

Question

I'm following these two tutorials: (1) LearnOSM and (2) OSMGWC. They cover OpenStreetMap, PostgreSQL + PostGIS, and GeoServer.

I've downloaded an extract of a state from extract.bbbike.org. I have all the layers configured together and the map looks perfect except for one thing: there's no surrounding ocean tiles. It's all white.

I'm thinking I could just replace all empty tiles with water tiles, and that would solve my problem. How can I do that? Or, if my idea sucks, what should I do instead?

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Answer 1

I ended up just downloading water polygons for the whole world and putting it at the bottom of my layer group. Worked out fine.