The army in each country could man the forts and operate the mines, and a special corps of the army could even operate the submarines, which (if their only office is to prevent actual invasion) need hardly leave the "three-mile limit" that skirts the coasts.

If the people of any country do not care to have dealings outside; if the nation is willing to be in the position of a man who is safe so long as he stays in the house, but is afraid to go outdoors, the problem of national defense is easy.
But if the people desire to prevent interference with what our Constitution calls "the general welfare," the problem becomes exceedingly complex and exceedingly grave--more complex and grave than any other problem that they have.

If they desire that their ships shall be free to sail the seas, and their citizens to carry on business and to travel in other lands; and if they desire that their merchants shall be able to export their wares and their farmers their grain, also that the people shall be able to import the things they wish from foreign countries, then they must be able to exert actual physical force on the ocean at any point where vessels carrying their exports and imports may be threatened.

Naval ships are the only means for doing this.
The possibility that an armed force sent to a given point at sea might have to fight an enemy force, brought about first the sending of more than one vessel, and later--as the mechanic arts progressed--the increasing of the size of individual vessels, and later still the development of novel types.
There are two main reasons for building a small number of large ships rather than a large number of small ships.

The first reason is that large ships are much more steady, reliable, safe, and fast than small ships.