New Player question - taking out Oceania

[Piet]

My first game - I play Abysia against Oceania, C'tis, and Agartha on a small map. I do fine taking out and taking over all land areas. I learn to empower a couple of my mages to have water skills and figure out how to make water breathing ring. I have a couple of low production amphibian provinces, and keep pumping those armies into the Oceania empire. But my brute force approach is not enough... I can't bring my armies with my water breathing leaders and am scared to commit those mages that have developed water skills.

What did I do wrong? What can I do to finish this map? Is there some way that I can make my armies be water breathing?