If you look in https://valheim-map.world/ and generate a map, put the "Visible Layer" setting on "Biomes Only", you'll see that the water area around any land mass is called "Shallows". The Valheim developers just haven't added code to the in-game maps to say Shallows when you're in shallow water. Instead it still says the type of land that's under the water. The actual Ocean biome however is coded to say so, and sometimes does come close to shore.