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Regeneration In Process
The point is that seapower should be comparable, if not superior, to the army in terms of usefulness, at least for a large part of the world.
Depends on time and space.

Anyway, two points:
- Building up a fleet. You could generally either commission privateers or confiscate trade ships for your own use; you don't have to actually build a fleet and many people didn't;
- River flotillas. Those tend to be badly ignored, but they were very important in regions with large rivers. Don't recall exactly, but I think Assyrians used them while campaigning against Marduk-apla-iddina II the Incredibly Annoying? River fleets figured prominently in Chinese civil wars (and in the Russian Civil War, though that last one is notable for some of the most bizarre weapons, tactics and techniques ever and so isn't a good example). River boats were at least as important to the (early) Cossacks as horses.