Magic:
EA R'Lyeh is a very heavy user of water and astral magic, less so in death and earth, and a rare small amount of nature.
We start with the Gorge of Ancient Cities, which produces the following gems:
3 astral
2 water
1 earth
Getting more sites quickly is always important. Starting research for us is usually Conjuration 4 for Voice of Tiamat. This was another of Evilhomer's recommendations that I fully endorse for EA R'Lyeh. It reveals all elemental sites in an underwater province that we control. This can give your gem economy a boost quickly. Conjuration 5 gives acashic Record, which some people love and others hate. If you like the spell, getting it right after Voice of Tiamat is not too hard of a decision. If you don't like acashic record, you can move to other branches after getting voice of tiamat. Conjuration gives some excellent summons, and we don't even mind the aquatic ones. Asp Turtles are downright nasty for example. And if you have enough death, you can get the ubiquitous Bane Lords or even tartarians if you squeeze out that much death.
As Belcarl pointed out, our Mind Lords normal ranged attack is enslave mind, so we don't need to push thaumaturgy right away, but it is a very important line of magic for us. All the Astral relocation spells (Teleport, Gateway, and Astral Travel) are from the thaumaturgy line. And the strategic mobility of these spells cannot be stressed enough. The threat of having 12 Mind Lords and their armies suddenly port almost anywhere can keep your neighbors honest.
Construction gives the essential amulet of the fish at construction 4, and no matter what paths you take, magic items are always things to hand out to important commanders (and not so important commanders if you have the time/gems available).
I normally leave blood for last. No blood hunting underwater is a major hindrance for fully exploiting this path. And we have no national mages that have access to it.
Spells I try to push for as EA R'Lyeh, depending on magic paths available:
Antimagic (Lobo Guards have lousy MR, and our other slave troops are below average as well)
Voice of Tiamat
Bane Lord
Gift of Health
Maelstrom
Arcane Nexus (Although I would be wary casting this in an MP game)
Teleport
Gateway
Astral Travel (Very deep in thaumaturgy, so hard to actually 'push')
Arrow Fend or Storm (depending on air available and what casters I have in the field that can cast these)
Communion Slave/Master
Returning
Personal Luck
Luck
Battle Fortune
We start with Twist Fate, and there are other spells I love to get, but take very heavy paths so I do not 'push' them so much (i.e. Call Abomination).
Amulets of the Fish are your land force bottleneck normally. Getting indy mages with air and water or empowering mages with one or the other can help cut this bottle neck down some. Normally only the pretender can make these and only if you have them available to do so. A lot of our pretenders and our 2 more powerful national mages lack hands so cannot benefit from dwarven hammers. Forge of the Ancients can mitigate that somewhat.
Nethgul is a ton of fun to give to a Slave Prince. He often gives his bearer etherealness though spells and does a large amount of horror marking, soul slaying, and mind enslaving at the enemy. I can't even remember the number of times I'll notice that I now have horror-marked troops that used to be enemies. If only he would mind enslave
before horror-marking.
Mind Lords and Aboleths can only carry 2 misc items, and this will have you gnashing your teeth as they cannot not even don a simple astral cap! Being handless means they cannot use some of the excellent path boosting weapons. The first item to enable them to go landward is of course the amulet of the fish, but the second item is less cut and dried. Ring of wizardry is very nice, but extremely expensive. Amulets of missle protection aren't bad if you don't have any field commanders to cast arrow fend, or have a commander with hands (I know it sounds odd, doesn't it?) to hold a staff of storms. I'm loathe to hand them supply items if I need them as the 1 slot is just to valuable for such an item. I hand the +supply items to less important sub-commanders, like slave priests.
Clam of Pearls should be high on your magic item list for researchers. Getting a lot of these can really boost your astral income. It takes them awhile to pay for themselves, although less time is needed to 'break even' if you have access to dwarven hammers (Accursed hands! How you elude us!), access to sites that cut the construction cost of items down, and/or forge of the ancients. Even without all these, the clams are welcome.
Slave Princes are ok thugs but their MR is only 12 to start, making them less effective as SCs. A magic resistance amulet goes well here. I like to give 1 of them the Trident from Beyond and Nethgul. Then I empower him to water 1 (for the trident's water bonus) and customize his other items as I see fit. He can lead a fair chunk of non-magic troops by default and is amphibious (and he has hands!) so can lead raiding forces out of the water if you so desire. Shields are always highly recommended for any thugs/SCs, but wavebreakers are decent weapons and are very thematic. The aforementioned Trident from Beyond is another such weapon.
Once your conjuration is high enough, the ever-popular Bane Lords make for excellent SCs. You really can't go wrong with these guys. Just make sure to outfit them well from your item treasury.
Tartarians can be really nasty, and if you have Gift of Health up, they can slowly heal their afflictions and be real powerhouses.
Call Abomination is costly, but these monsters are well worth it. Being amphibious, regenerating, huge, and nasty. I have to try giving one Gift of Reason to see how that works out.