I'm sorry, but I'm not changing that detail. I feel water is workable, especially for the more defensive nature of a Mayonite Paladin, and also makes sense for Mayon domain as a Moon Goddess, as opposed to lightning.

If you want to make a Mayonite Paladin, that's how a Mayonite Paladin would work. Even if you do use a smaller shield they're still going to have water-based enchantments, most likely on the shield as opposed to anything else.

Edit: Since you edited while I was posting, Ice does work.