ok
now, keep in mind that there are a bunch of qualifiers i use wrt sea unit builds. so i'll try and address them as i go along. just stuff specific to our scenarios and it might not be everyone's cup of tea. however, it has worked very, very well for us and it is imho the best use of sea unit behavior for civ3 (considering our circumstances that is).
the main point regarding "diversity" is to have two (2) or more separate regions on the map. by this i mean that there's 'isolated' regions on the map that have two (or more) distinct resource access areas.
for example, Great Britain can build all types of ships, including capital ships, in the home islands (due in course to a resource called 'Industry'). this list includes TBs, DDs, PCs, CAs, CLs, SSs,
BBs, BCs, CV/CVLs (bolded = capital). however, abroad (in the colonies and commonwealth holdings) they can only build non-capital ships due to the lack of the so-called 'Industry' resource.
of course, to maintain this 'resource isolation', one must set up the map to curtail (or remove altogether) sea/ocean/air trade. now, it doesn't haven't to be removed entirely. you could allow for it in certain cases through special city imp's or wonders and thus allow for
some construction of capital ships. however, that discussion is not so important here. the main point is that there are distinct regions on the map where two (2) sets of ships are able to be built: all (capital + non-capital which is typically the highly industrialized areas of the map) and the non-capital classes (abroad assuming that the requisite resources [e.g., coal and iron] are inside the strategic resource box of the cities).
my experience with actually trying to get the AI to build more seas units tells me that i had to give it a reason to build them...sounds simplistic
i know. but the point is if there's good reason for the AI to build them, then, in my experience, it will. good ways to promote this:
cheap prices - low shield costs for seas units is absolutely a plus for the AI. we intentionally set the prices for seas units in our projects real low and the AI does indeed build more than what we were originally accustomed to. don't have a specific formula on prices...but each class was sort of tiered out in terms of costs (e.g., BBs set at a certain tier for gen1-->gen2 and so forth).
easy access to imp's that produce vet sea units - we've seen that the AI is far, far more inclined to build sea units if it has the ability to produce veterans. so granting an easy route to build these types of improvements is paramount irregardless of whether there's resource isolation. i mean, it is probably Issue No. 1 for the AI in terms of whether
to build or not to build, that is the question
escort - we all know how the AI loves to send along big-ass warships to escort their puny little transport units. annoying as hell imo...well, why not try and capitalize on this behavior and and design, a class of ships able to keep up w/ the transports? when set up in conjunction w/ the 'isolated resources', the AI does pretty well in building, say, cruisers, to tag along w/ their transport units.
ultra cheap transport costs - dirt cheap w/ no resource requirements. bumping up the transport capacity seems to work well too although we don't go too wild w/ this. however, the AI will build lots of them if they're super cheap. and as a result, amphibious operations generally increase. and the AI, if it has the proper infrastructure in terms of shield production and cities, will send along escorts (not always but the AI will definitely send them along if it has them).
raw materials - this is pretty much exclusive to my stuff. it's the reverse capture the flag method and there's a building available overseas (i.e., only in the colonies) that produces these units and they need to be shipped back from abroad to the home country in order to be cashed in for VPs and gold. the AI will guard these TRs if it has the inventory (see above). in addition, this method, while not for everyone due to the tediousness of it all, creates
de facto shipping lanes and it gives rise to another aspect of naval strategy: commerce raiding. this is a pretty neat tactic in AoI and CLs and other smaller vessels most definitely come into play here and the capital ships are at times (the early gen ones) at a disadvantage due to speed (mvmt points). anyhow, just a neat little side game that is present for AoI.
upgrading and obsolescence - we set all capital ships on an obsolescence track (i.e., gen1 BB is obsoleted by gen2 BB and so forth). this means that they do not directly upgrade to one another but the obsoleted version disappears from the build queue when the next generation is available. this is done in the editor...but the unit action to actually upgrade is not checked. the reason for obsolescence is not to penalize...but to put these capital ships on a different track compared to the non-capital ships which
can upgrade. upgrade sequence can depend on a bunch of things and is pretty subjective. however, we set DDs, mostly, to an
every-other-generation path (especially when there's a lot of generations - 7 or 8 in AoI alone). CLs we set to upgrade directly to one another
in sequence. other cruiser types and submarines have varying upgrade paths depending upon the quantity of generations, quality of the construction, and some other factors. but again, subjective to a certain degree. but the point to take away from this is that the non-capital ship generation lines have a nice 'finish line' so to speak whereas the capital ships, with their brute force and all, do not. and the AI does upgrade its ships so long as it has the scratch and the wherewithal (i.e., imp's that allow for it). the obsolescence also puts the human player into decision mode where he/she has to decide whether to keep the older generations of capital ships around for secondary roles or whether to scrap them (disband).
another less-discussed factor is the stat arrangement. i won't get into our formulas here b/c it's dry and pretty technical (yet, for us,
extremely effective). however, what it accomplishes is a few things: it gives each type of ship the proverbial
raison d'être which is
imperative for all unit types. don't confuse the AI or give it multiple types of units that essentially do the exact same thing. differentiation is the key - and we use several things to set certain sea unit types apart from the others like total tonnage/displacement (i.e., HPs), total no. of guns on board (bombardment rating and RoF), torpedo tubes (very important for the TBs & DDs since they have high rates of fire but lower bombardment ratings [compared to the capital ships]), abilitiy to 'detect invisible' units like submarines, the 'invisible' tag for subs, extra movement points for cruisers, and a few other things. again, real technical stuff but it allowed us to get that differentiation that we were after. i mean, not only does the AI build all of the sea unit types in one location or another, it also gives the human player some options as well. for example, if the human player says go scratch to building non-capital ships, he/she is gonna have piles of obsolete units lying about at some point whereas construction of non-capital ships has a more fluid development path (i.e., upgrades allowed) and offers the player a more diverse fleet in general.
hope this may help some and always happy to provide more details as time permits