Well, about the name of Selbbub, it’s dificult to say that will be an correct name based on a legend or something, all the others Elder Dragons have names that are close to what they are based;
Zhaitan is “Shaitan” (Satan on the Islamic version), Jormag is based on “Jormungandr” (One of the sons of Loki, the serpent that revolves the world), Krakaltorrik being based on the volcanic island of Krakatoa, and Mordremoth is based of Mordred from the Arthurian legend.
I believe the name will not be solely based on water, but on something that resemble it (like, sirens, islands, Idk… I like to joke that it name would be “Saeren” because it resemble Siren, but that’s just an rp player dream)