[Alneyan]

I would recommend going on the offensive here: Ulm isn't a water nation, so they must rely on troops to protect their provinces, and won't be building forts beneath the waves. That aside, water provinces are much better for defence, since they offer protection against artillery spells (Murdering Winter/Flames from Afar), and getting in the water to begin with is tough; underwater nations also have a much easier time attacking you.

Most undead can go under water, so they are a worthwhile solution if you happen to have death gems lying around; other paths should have amphibian summons for the same purpose, but they will be fewer of them, I believe. If you want to bring in mundane troops, an air item can allow you to bring X troops with you underwater, and similar items exist in the Water path; you should really start building some Water magic if you are serious about invading the seas with hosts of troops. Raising Water to 2 can be done easily with Construction 6 and a Water-1 mage, and you should have recruited a mage with a Water-1 random by now.

Another solution is to build hit squads, that is commanders with little to no support, but a lot of firepower: you would then rely on Water magic to bring your commanders underwater, or Rings of Water Breathing (or any similar item). This is a cheaper solution than bringing all your troops with you, but commanders are vulnerable, of course. A last way of dealing with the underwater provinces is to bring the heavy guns: Sea of Ice will make it much harder to leave (or enter) the seas, and it might be just what you need. Of course, this is a Water enchantment, so it will be pretty tough to cast for you.