“In 1933, Lucia took injections of sea water as prescribed by Dr. Henri Vigres (Shloss 255), further identifying Lucia as a fish.

Silver-tongued “Selvertunes” (299.23) footnoted as a “fish” — a silver fish — is Lucia, the apple of Joyce’s eye.  “The lappel of his size” (314.33-).    In “eye smells fish,” fish corresponds to apple from Finn’s eating the salmon of wisdom (“Salmonson” (297.03)).   “Eye” is pronounced the same as “Ei” (egg in German) suggesting baby and eye — the expression “babies in the eyes” means “Love (Cupid) in the expression of the eyes.” (Brewer’s).    As Joyce indicated in his letter to Budgen, these footnotes are “by the girl” (Letters I. 405-6).   The adoring father says to his daughter Lucia:  silver  “fish.  That’s U.”  (Dutch U: you, and Chinese (yu) yü: fish.  (McHugh)). 

John Gordon points out the many associations between Issy and Lucia.  St. Lucia is the patron saint of light, vision, and the blind.  Issy is associated with double ‘i’s:  Iris, Isis, … . (Gordon 80)”

http://hjs.ff.cuni.cz/archives/v9_2/essays/tanaka.htm#_Toc207181796
From ‘The unifying themes of Finnegans Wake’ , an essay by Tomoyuki Tanaka