את זה תאכלו מכל אשר במים
Mikol asher ba’mayim, “from all that is in the water,” implies that an amphibian which has simanei taharah, signs of purity/kashrus, may be eaten. “A fish is a fish”; the question is only whether it is a kosher fish. Its status as amphibian is not relevant. The Mishnah in Meseches Keilim 17:13 appends this idea when it says anything in the water (utensils made from amphibious creatures) are tahor (utensils are considered tahor, ritually pure) except for the kelev ha’yam, sea dog (possibly the otter), since it flees to dry land.” In other words, a sea dog maintains the status…
