I'll tell you the situation that prompted me to think about lethal sea bombardment for field artillery. I was on a continent with a small inland sea, only about a couple dozen squares, but large enough to have harbors, ports, etc. I set up a city near a coastal barbarian encampment, it died, but the associated pirates remained, and in one of the sea spaces for my city. I really didn't want to build a ship that would serve no further purpose, so it took until mechanized warfare before I could bombard it to death.