The Torah in two places (Sefer Vayikra Perek 11 & Sefer Devorim Perek 14) lists the two signs necessary for a sea creature to be eaten: It must have fins to swim with and scales on its body like armor (See Rashi in Vayikra Pasuk 9).
Rashi there (Pasuk 12) brings a Medrash Toras Kohanim which derives from this Pasuk (12) that sea creatures are kosher even if these signs are not on them when the creatures come up from the water, as long as they had the simonim while in the water. One can presumably assume then that there are such creatures who this applies to - they have the kosher signs while swimming in the water, but lose one or both of the signs while coming up. How many species does this apply to?