Building temples a little ways in from the shore can be helpful on occasions, though nowhere near always.
1). Reduce freespawn
2). Force them to spend money on their own temples, giving you something more to raid, and them to defend.
OR Force them to come up onto land to go after your temples, where you should have an advantage.
3). Waste your money.
4). Waste their money - underseas aren't always rich.
Remember, every temple is a dozen Illithids, or something like that.
I know you're looking more for tactical advice, but I'll go as vague as possible, for the moment. I'm sure someone will bring something specific to the table ASAP. I'll go the irrelevant route of SunTzu
Quote:
20. Hold out baits to entice the enemy. Feign disorder,
and crush him.
21. If he is secure at all points, be prepared for him.
If he is in superior strength, evade him.
23. If he is taking his ease, give him no rest.
If his forces are united, separate them.
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Quote:
3. Thus the highest form of generalship is to
balk the enemy's plans; the next best is to prevent
the junction of the enemy's forces; the next in
order is to attack the enemy's army in the field;
and the worst policy of all is to besiege walled cities.
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Quote:
IV. TACTICAL DISPOSITIONS
1. Sun Tzu said: The good fighters of old first put
themselves beyond the possibility of defeat, and then
waited for an opportunity of defeating the enemy.
2. To secure ourselves against defeat lies in our
own hands, but the opportunity of defeating the enemy
is provided by the enemy himself.
3. Thus the good fighter is able to secure himself against defeat,
but cannot make certain of defeating the enemy.
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Quote:
11. If we wish to fight, the enemy can be forced
to an engagement even though he be sheltered behind a high
rampart and a deep ditch. All we need do is attack
some other place that he will be obliged to relieve.
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Quote:
16. The spot where we intend to fight must not be
made known; for then the enemy will have to prepare
against a possible attack at several different points;
and his forces being thus distributed in many directions,
the numbers we shall have to face at any given point will
be proportionately few.
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There are specific things you can do to aid in underwater expansion, but in general, the advantage will always lie with the underwater nation.
I'd get water breathing ringed scouts underwater, attempt to draw the enemy in one direction, and then strike with a force that is smaller, but much stronger on an individual basis. Pillage, overtax, tear down any structures you can, then flee. Once the water nation actually starts looking weak, then consider a serious war.
One practical possibility is Skeletons and Revenants/Mound Kings. They won't resemble a serious fighting force, but you can assemble them in large enough numbers for a low enough cost to harass the enemy on one front, and supplement the small amount of amphibians you can recruit/summon from a land site.
Without a specific nation in mind, you'll have to pour over the spell appendix in the manual for spells castable underwater. There ARE a few available even as Abyssia.