It could be more simple as you suggest, but I want a better solution. For instance a separation of late industrial torpedo boats / dreadnoughts and WW2 era destroyers.
I thought about how to tackle the problem of naval combat.
The problem is with naval combat is that there is no equalizer like siege collateral
or any sort of flanking bonus and things like that.
Naval combat is akin to smashing your head against the wall and seeing if the wall has a dent or if your forehead does.
Problem #1: Ships are too expensive for what they do; which is generally very little, and very niche. The cost-benefit ratio is unfavorably skewed.
Problem #2: There aren't enough classes of ships to lend variety to combat.
Here are some of my mad ramblings on something I came up with:
TAGS:
(L): Light-class; meant to overwhelm in large numbers; sea-based equivalent of siege.
(T): Transport-class; self-explanatory
(BB): Bread & Butter; meant to form the core of your forces
(F): Flagship; muscle ships, think of them as cav-equivalents or CR melee units on land.
(S): Support-role; bag of tricks
MAN TIER
Lembos (L) 2 STR, 3 MOV, 30 Hammers, Inflicts collateral damage (max 40%)
Galley (T) 2 STR, 2 MOV, 40 Hammers, Can carry up to 2 units.
Gain an extra MOV point for each Worker or Slave onboard.
Trireme (BB) 3 STR, 2 MOV, 45 Hammers. May bombard (6%). May blocklade. Can inflict flanking damage on Galleys upon sinking a unit.
SAIL TIER
-All Sail Tier units gain a +50% bonus vs. all ships of a preceding tier class.
-All Sail Tier units may enter ocean.
Caravel (S) 4 STR, 5 MOV, 45 Hammers, Can carry Scouts/Explorers/GPs.
May pass through rival territory. No maintenance upkeep.
Galleon (T) 5 STR, 4 MOV, 55 Hammers, Can carry up to 3 units.
Gain an extra MOV point for each Worker or Slave onboard.
Frigate (BB) 7 STR, 4 MOV, 60 Hammers, Can inflict flanking damage on Galleons upon sinking a unit.
May bombard (8%). May blockade.
Ship of the Line (F) 8 STR, 4 MOV, 75 Hammers, +50% vs. Frigates & Galleons.
Priority targeting of Frigates in an enemy stack. May bombard (8%). May blockade.
Privateer (L) 5 STR, 5 MOV, 60 Hammers, Inflicts collateral damage (max 40%)
Hidden nationality. May blockade.
Harbor: +1 Food from water tiles. +3 EXP to all naval units built in this city.
Lighthouse: +1 Commerce from water tiles. Heal naval units extra 10% damage/turn