According to the dubious Wikipedia*:
After he was revived yet again in the 1960s by Stan Lee and Jack Kirby, Namor demonstrated powers of various sealife that had not been shown in earlier stories. However, an editorial note in Marvel Tales #9 (July 1967), stated explicitly that "nautical Namor has since lost his power to imitate the characteristics of fish..." According to one of Sub-Mariner's scripters, Roy Thomas: "As for Namor's electrical and other sea-creaturely powers. They were used in one or two stories in F.F. and the Human Torch series in Strange Tales, then dropped - as one of Stan [Lee] and Jack [Kirby]s early (and quite forgivable) mistakes. The explanation, given in a reprint of a Torch/Namor bout a few years later, was that he had these powers only for a short time and then lost them."
To answer your question, it was just a limited characteristic of his abilities during the Silver Age comics.
*I consider everything on Wikipedia to be at least partially suspect.