J-Word Play #27

Here's one that I hope won't be too easy for you:

海にもいるけど、空も泳ぐ生き物はなんでしょう？
(うみにもいるけど、そらもおよぐいきものはなんでしょう?)

If you think you know the answer, shoot me an email at blueshoe [at] jadij.com