I hope the origin of mister waterbender's power will be explored. Will it have been from the sea, or did he instead get it from a lake? How will that pan out with the eldritch lady's blessing on Julian?
And is Julian hoping to introduce Mister to the sea monsters? I hope that's the case. That would be so fun.
It's been implied that his powers might come from being passed on by blood. Emperor lived for centuries and could possess other abilities. It's possible he's strong enough to pass on a fraction of his supernatural power into his children.
That means mister is kinda like Julian, he got blessed by powerful being (Emperor) but have never encountered spirits and monsters in his life. This can lead to prince being viewed same way Julian was by the Forest spirits, Anathema or enemy who walked into thier domain.
Prince has power over water he sees and within his body, but not the favor and protection of the domain of the sea and all it encompasses like Julian who seem to be unable drown in the sea water, he will never get lost, fish will get into his nets on thier own etc.