Agreed.
But even for its current limited use the naval unit tech progression is inadequate. If I remember correctly, a Trireme can
only be upgraded when you get to Frigates, which is
three eras down the line. That's going from 6

to 30

! A trireme is pretty much wiped out by a Crossbowman in the middle ages: there really should be an intermediate coastal defence ship capable of escorting a Medieval fleet on a coastal/island-hop.
I would suggest some sort of War Galley, appearing somewhere in the Medieval era, with combat strength around 15 (same as caravel) and ranged attack 12 (same as crossbowman). This would make the caravel a purely explorative unit, compared to a warship. The War Galley would still be restricted to coasts, but provide an intermediate between Triremes and Frigates.
Then the Turtle Ship could be a souped up War Galley, and while its usefulness could still be debated, it at least wouldn't preclude intercontinental exploration for Korea.