Dealing with Ocean Nations

[NTJedi]

Zonso said:
...
They have huge armies and can invade several of my territories at will and I can do little but respond afterwards.

... utterly destroyed the next turn by their combined 400+ armies!

... Worse now is Oceania has built Forts right next to my land provinces.
...
Any advice or strategy? Thanks.

Hopefully you have some death gem income... the undead armies can be gathered in great numbers and most undead can travel underwater. If you're stuck with only nature and earth gem income you'll have to focus on mechanical men.
You'll need time to build strong armies to march into those waters... hopefully you can hold your borders. SCs are an option as well yet make sure you can protect yourself before stealing underwater enemy provinces.