I think "ñuhor" is correct rather than "ñurho" here.
https://wiki.dothraki.org/High_Valyrian_Adjectives#Class_I see singular accusative aquatic.
Going just off the tables, I'd say you're right. I also know that there are a lot of instances with stem letters that come back in weird positions, in some weird declensions... For instance the class one adjectives like nuha, bisa-type specifically, when used with aquatic nouns, goes from bisor in the acc to bisro in the genitive. It shows the precedent and I don't know enough yet to say either way. All I can say is that there are a couple other things in the lessons that I think might be wrong so it could go either way. The course creators and subject matter experts are going to have to see.