How does one fight the sea?

[Dinedel]

I'm a very new player, so please excuse my inexpiriance. I am currently playing a small game in the Middle Ages with one of my friends, whose nation is R'lyeh.

I am playing as Machaka, and despite having the upper hand at the moment, I have no foreseeable method of getting under the sea and attacking my foe directly. Instead, I suffer constant raids on my sea facing provinces.

What can I do to get under the water? As Machaka, I'm having a really tough time getting water magic, and I regret not taking it with my pretender, a lesson I didn't learn until later.

Do I need to forge items that let my commanders breathe underwater? What about my troops? Do I need to summon troops and commanders that are capable of fighting under the sea? How do I get the needed water gems if this is the case?

Thanks in advance for the help.