Why not use this as an opportunity to buff forts? Let coastal forts next to a city make that city count as coastal. Doesn't use that weird Civ 6 system where cities build improvements and there's the opportunity cost of building a fort on a tile that could have literally anything else on it, but in exchange, you're making a city able to have a harbor, wharf, lighthouse etc, that otherwise wouldn't be able to have it. Coastal would still be vastly preferable, but I imagine players may find scenarios where it outperforms a cottage, windmill, or watermill.