There are other magic items to give water breathing abilities to your armies; however, I think the other items require more than Construction 4, and so cannot be forged in the demo. I had forgotten about the global enchantment which allows *all* troops to go underwater as well. It is called Thetis' Blessing, require 5 water, and Enchantment 9 (making it a end game spell, and unavailable with the demo). There is another similar spell which prevents travel between land and sea, thus forbidding Atlantis from invading you (it requires Alteration 7, and so cannot be used in the demo).
If you are interested in knowing more about summoned creatures and/or magic items, there are two PDF guides showing everything that can be obtained during the course of the game. However, viewing these files is unadvised if you would rather discover spells on your own. These guides are linked to on a couple of websites, including Arryn's available
here.
Edit: There are also a couple of magic items which enable a commander to breathe under water. Among these are the Shambler Skin Armour (requires Water 1 and Construction 2) and the Amulet of Breathing (Air 2 and Construction 2). It might be enough if you have a powerful commander able to take provinces on her own.