Reference for Mikvaot 6:5
הַשִּׁדָּה וְהַתֵּבָה שֶׁבַּיָּם, אֵין מַטְבִּילִין בָּהֶם, אֶלָּא אִם כֵּן הָיוּ נְקוּבִין כִּשְׁפוֹפֶרֶת הַנּוֹד. רַבִּי יְהוּדָה אוֹמֵר, בִּכְלִי גָדוֹל, אַרְבָּעָה טְפָחִים. וּבְקָטָן, רֻבּוֹ. אִם הָיָה שַׂק אוֹ קֻפָּה, מַטְבִּילִין בָּהֶם כְּמָה שֶׁהֵם, מִפְּנֵי שֶׁהַמַּיִם מְעֹרָבִין. הָיוּ נְתוּנִים תַּחַת הַצִּנּוֹר, אֵינָם פּוֹסְלִים אֶת הַמִּקְוֶה, אֶלָּא מַטְבִּילִין אוֹתָן וּמַעֲלִין אוֹתָן כְּדַרְכָּן:
Regarding a crate or a chest [floating] in the sea, one may not immerse [objects] in them, unless they were pierced [with openings] the size of the tube of a flask [connecting their waters to the seawaters]. Rabbi Yehuda says: for a large vessel [the opening must be] four handbreadths, and for a small vessel [it must be] the majority of its size. If it was a sack or a basket [floating in the sea], one may immerse in them as they are, because the waters are mixed. If they were placed beneath a pipe [such that rainwater flowed through them and into a <i>mikveh</i>], they do not invalidate the <i>mikveh</i>; rather, one may immerse them and take them out normally [without worrying about the water in them rendering the <i>mikveh</i> impure as drawn water].