I agree that melee ships need some love and some possible rethinking.
Ignoring UUs, the Trireme is fine (maybe a little weak), the same with the galleass. The renaissance/medieval is when it starts becoming odd and it is mainly the caravel I think is bit odd.
I'm fine with it not being very strong as it is more of a scout but then why is it melee? Either it needs to be something like a glass cannon where it is fast and has good offensive, or it needs to be ranged in my opinion. I imagine navies where the ranged ships are lighter and provide support and some weakening barrages as the melee ships which are stronger roll in and finish the job but I guess that is just my own opinion. Some people (and reality) probably favors the fact that the "big guns" are ranged while the weaker ones are fast and get in close.
The frigate is fine in my opinion although if you refer to my previous paragraph I'm thinking maybe it should become the new melee ship instead of caravels. I don't remember where privateers come into play, but if they are about the same time as frigates that opens an interesting scenario.
I think then than caravels could be scouts with weak range, frigates your main range, and privateers your melee. However privateers are supposed to be somewhat glass cannon like and sneaky as far as I am able to tell with the capturing units and whatnot, so this may make them lose their flavor.
I love the ironclad but I honestly think it doesn't fit in very well with the progression. Perhaps it should be an upgrade for the privateer? Basically we have a unit that is supposed to be very tough for its tech level. I am quite ok with with it being melee and I love the coastal bonus/ocean penalty it has. The problem is where it does it fit in? The caravel which is melee logically upgrades into the iron clad, but even if we ignore my earlier points, the caravel is basically a light scout that does melee, while the ironclad is a slow (outside of coast) tanky unit. It just seems odd. You can have the frigate upgrade into the iron clad, but then it turns into a melee unit (which may be ok if we changed the caravel to range and the frigate to melee) but then you are left without a deep sea tank except the privateer, which isn't really a tank.
The problem basically isn't too much different in the modern era with battleships, submarines, carriers, and destroyers. Refer to my comment about big guns to see how I feel about ranged and melee. The problem here is that we basically have battleships as ranged, destroyers as melee/scout/support (interception) hybrids, submarines as ranged glass cannons, and carriers as support (depending on the aircraft obviously). Before we even discuss the individual units I think most of us can agree we need a ranged, a melee, and a support. The others are fun and important, but they aren't essential and this era is tricky. Let's start with the battleship:
I think the battleship works pretty well as it is being ranged. The problem arises when if we try to change other units like the destroyer, we are left without a melee and then I would say this unit makes the most logical melee choice, but then again the big guns are begging to be use for bombardment.
I like destroyers, but once again I think it is a bit odd they are melee. I think their role of submarine hunter and interception support is great, but aren't support units usually either scouting or in the back, not going into melee?
I LOVE submarines! I think they are implemented quite well and there really isn't much I would change about them except perhaps making them a bit weaker. Their ranged attack is ok, but I think if you find a submarine you should be able to kill it pretty easily. That being said, I sometimes think 3 range for a sub is too much but taking it to 2 might be overkill. Either keep it 3 and nerf the ranged just a hair and the strength a little, or make it 2 and keep the sub the way it is.
The carrier really doesn't need much explaining in my opinion. Obviously you make it what you want it to be, depending on your aircraft and I can't think of anything to add to this. I think it might be interesting to give them interception (most carriers are equipped with that sort of weaponry) but then a fleet with a destroyer, a carrier, and a just 1 fighter not to mention 3, is pretty much untouchable from air (assuming fairly equal numbers). So while the idea might fit, if we were to do that the destroyer would need to lose its interception. I personally think that would work well and better define the roles. The carrier could become more of a support, destroyer a melee sub hunter/scout, battleship your general range. and subs your anti-naval units.
Once you reach the atomic (I think?) age you add some more units to the mix and it almost gets convoluted. With all the previous units, you now have missile cruisers and nuclear subs. The problem is that units overlap, and while that isn't necessarily a bad thing, it does make things a bit confusing.
The missile cruiser as an indirect ranged unit carrying missiles makes perfect sense. I think the interception almost makes it a little too strong, but I can see why it has it considering I imagine it carries SAMs. As a suggestion, I think maybe we should increase the missile count (5 perhaps?), maybe even you can buy missiles on the cruiser, but take away the ranged attack. It might make the unit a little underwhelming but I'm just trying to give each unit a defined role.
As for the nuclear sub, I don't really know what to do. Obviously it is much like the regular submarine and that is fine. That it carries missiles makes it overlap with the cruiser, but I at least see logically why it happens. Perhaps make it only carry ICBMs so you can stack your cruise missiles with the cruiser? Another problem is that is overlaps with the regular submarine. Obviously it is a bit stronger and can carry missiles, and the bonus against other subs is odd to me, but not a big deal, but then why do we have this as a separate unit instead of an upgrade? I propose making the regular submarine upgrade into this unit and I think that fixes this issue. (To be honest, this might already be the case and I just forgot!).
Bottom line: Even if nothing else changed, melee ships should receive a slight buff. My other suggestions I think make sense and would make the game have more direction, but there are also more pressing issues (such as the religion UI!).
So there you go, if you made it this far, congratulations on having an above average attention span and I hope we all can get some good discussion stemming from this!