I think the Harbor district is good as is. The trade route slot is quite valuable.
I agree. Harbor district may not be super high priority, but the additional housing, food, production, gold, and especially trade-route make it much more appealing than many other district options.
Of course this is besides the point, since you can have a 95% land-based city and still build the harbor district so the district alone isn't giving incentive for players to place their cities actually upon the coastline.