They came from the SEA?!

[HoneyBadger]

One solution to consider as a partial fix is to recruit as many archers as possible in your shoreline provinces, since the water nations don't have much in the way of ranged attacks, and I don't think any missle troops-atleast in EA. It won't solve the problem, but it will give you an advantage, esp. if you can afford a commander casting fire arrows. Combining them with calvalry scripted to hold-attack should also help, particularly against EA Atlantians. Illithids are tougher, try mass-producing amulets of anti-magic and bows of botulf, and handing them out to cheap commanding officers. Shamblers on coasts seem to be common. I usually know where atleast one province with shamblers or ickthys are by mid-game. It takes a big investment of time and money though to really mass them, so if you find a province like that early in the game, grab it and hold on, and don't shirk on buying the troops each round, if you can help it. A hundred+ shamblers just sitting around collecting upkeep seems like a waste, but they're worth their weight in gold by the late-game. And ofcourse, who says you have to let them sit? There's no wrong time to invade the ocean, except when it's too late.