"Kesi jevi gelti issi?"

Translation: Are these your helmets?

[emzo99]

Tips and notes doesn't mention this, but how do you decline īlva and jeva in aquatic genitive? īlvro/jevro?

[ArminGali]

In this case the your is plural?

[Ctxkh1Me]

Correct, jeva is used for second person plural, aōha for second person singular.