Now, in order to transfer ground troops from one continent to another, they need to be transported by sea, while they are slow there, which is logical. My idea is to make a sea road that will be built by a special worker or some kind of unit. It would be like underwater rapid movement. Or learn how to build ordinary roads for units. 

Or you can also suggest that when a land unit is combined with a sea unit, it will move faster so that you can quickly transport troops from one continent to another.

Also, as an idea, you can create a special naval unit that could accommodate a certain number of ground troops and quickly move them across the ocean.

Since now there are options only with the airport, and the airport is being built only on its territory.

These are just some ideas that could help ground forces move faster across the ocean.