Quite early in the Cyclopedia, starting on page 10, we have the Religion in the Realms section. The authors give an overview of the main deities, then talk about some not so main deities. Among those latter, we get a short section on the Elemental Lords: Grumbar, (God of Elemental Earth;) Kossuth, (ditto of Fire;) Akadi, (Air;) and Istishia, (God of Elemental Water.)
Now, the idea of Elemental gods, kings, or what-have-you goes back quite some way. At least to the renaissance and Agrippa's Three Books of Occult Philosophy. The idea shows up on more that one occasion in European Occultism and there are several variations.
Back to the Cyclopedia. Page 18 gives a chart with Classes and the typical gods they worship, then a smaller list of some professions and the powers they sometimes venerate.
For Sailors, we get the following list:
"TYMORA, SELUNE; Placate STRAASHA, MISHA, TALOS, UMBERLEE."Hah!
Straasha is, of course, the Elemental King of Water from Michael Moorcock's Elric series. I can only assume they meant it to be Istishia. Whoever was responsible for that chapter may have substituted what was, to him/her, a more familiar name for the concept.
Or, in Ed's original campaign, he may have ported over Mr. Moorcock's version of the Elemental Lords and the writer/editor gaffed in not correcting the reference in the "Official TSR Version" of the Realms.
Not really surprising. Several of the Forgotten Realms deities come from other mythological sources.
And I could hardly fault Mr. Greenwood for swiping ideas from Mr. Moorcock, for the sake of his home campaign. Many of us did so!