Many aquariums now feature a plexiglass walking tube, so you can see the tanks not from the top, or the side, as was traditional in the past, but from the bottom. The fish glide about over your head. It's really quite majestic. And then I heard a restaurant is doing this too, serving the food in a plexiglass tube in the ocean, where the fish glide over your head as you eat....other fish. I won't be dining at this restaurant anytime soon, as it's half a world away from me and charges well over $200 per meal. It's some fancy fish.
But this gave me an idea for something...interesting. We would build a tunnel, with plexiglass, between two islands. Let's start with the Dominican Republic and Puerto Rico. A small electric tram runs through the tunnel. When customs approves your trip (because this would cross an international border), you get on the tram, and the tram goes into the dark tunnel. Then there is light: you are in a plexiglass tunnel, with the fish idly swimming above your head. The glories of the ocean are all around you, and it is quite arty. After some time of that, then it's dark again, and then you're at the other island, where you disembark. Visitors approved for the trip in the other direction now board the tram.
Further tunnels would go to other islands from here, as there is a chain of islands that eventually reaches Venezuela. Then I would look into other potentially interesting connections. A cross red-sea connection, from Yemen to Ethiopia, perchance? A visible chunnel, from London to Chalais?
The technology would only allow for short tunnels. Tsushima to Busan would be doable. New York to Brest would not. (The deeper part of that tunnel would collapse under the immense weight of the Atlantic, and the entire tunnel would flood.)