Thing is, when I'm building navy I'm making the choice of NOT building land based military units. I need a very strong reason for this. Pillaging fishing boats is not a good enough reason. Bombarding enemy defenses is not a good enough reason (might as well build an extra catapult). They're not even that effective as fishing boat protectors, since it's generally cheaper to just replace destroyed fishing boats with new ones after the war, rather than maintain an expensive navy that might not be successful against a navy-building-crazy AI anyway.
On Archipelago maps it's a different deal obviously. Otherwise, I normally only build navies for one reason: troop transport, and even then I cringe at the thought.