There's a lot of water that we can use in-game, and humans have made artificial islands and other large water related structures so I'm hopefully not going too crazy with the ideas I have here. I know that humans don't historically do much infrastructure with water, but I'm sure we can get something going with some creativity 
*Additional effect from Steam Power
Allows Harbors (and districts which replace them) to be built further out at sea, away from the coastline.
*Additional effect from Plastics
Allows Airports (and districts which replace them) to be built out at sea.
Fishing Village
<Available at Naval Tradition>
An improvement which can only be built out at sea. A Fishing Village provides a small amount of housing and also grants food for each sea resource adjacent to it when worked.
Water Bridge
<Available at Engineering>
An improvement which must be built adjacent to a source of water and cannot be built on hills. A Water Bridge counts as a source of water, adds the "River" terrain modifier to its own tile and can be traversed by land units and sea units.
Offshore Resort
<Available at Capitalism>
A district which can only be built out at sea and cannot be built along the coastline. Offshore Resorts have high Production cost and high Gold maintenance, but they generate some Tourism and Amenities on their own. Offshore Resorts do not have adjacency bonuses. When a city has an Offshore Resort, it gains access to the "Grandiose Experience" project. A Grandiose Experience project generates a large amount of Tourism and Amenities while it is active, and awards a large amount of Gold when the project is complete.
Explicit Advertising
<Available at Social Media>
An Economic policy. +100% Tourism output from Grandiose Experiences, and your trade routes generate +5 Gold for each Luxury Resource in the origin city that is not at the destination.
Ok admittedly the last one doesn't have much to do about water but it's tied in with Offshore Resorts
*Additional effect from Steam Power
Allows Harbors (and districts which replace them) to be built further out at sea, away from the coastline.
*Additional effect from Plastics
Allows Airports (and districts which replace them) to be built out at sea.
Fishing Village
<Available at Naval Tradition>
An improvement which can only be built out at sea. A Fishing Village provides a small amount of housing and also grants food for each sea resource adjacent to it when worked.
Water Bridge
<Available at Engineering>
An improvement which must be built adjacent to a source of water and cannot be built on hills. A Water Bridge counts as a source of water, adds the "River" terrain modifier to its own tile and can be traversed by land units and sea units.
Offshore Resort
<Available at Capitalism>
A district which can only be built out at sea and cannot be built along the coastline. Offshore Resorts have high Production cost and high Gold maintenance, but they generate some Tourism and Amenities on their own. Offshore Resorts do not have adjacency bonuses. When a city has an Offshore Resort, it gains access to the "Grandiose Experience" project. A Grandiose Experience project generates a large amount of Tourism and Amenities while it is active, and awards a large amount of Gold when the project is complete.
Explicit Advertising
<Available at Social Media>
An Economic policy. +100% Tourism output from Grandiose Experiences, and your trade routes generate +5 Gold for each Luxury Resource in the origin city that is not at the destination.
Ok admittedly the last one doesn't have much to do about water but it's tied in with Offshore Resorts
I would be down for more sea resources for sure though. Atolls would also be nice, not sure why those were removed.
