Mishnah Taharos 4:8: The uncertainty of an impurity floating on the water's surface [where it is uncertain whether or not someone came into contact with the floating object: this was declared pure by the Sages] whether in a vessel or on the earth. Rabbi Shimon says: in a vessel it is impure [since it is in a fixed place], and on the earth it is pure. Rabbi Yehuda says: If the uncertainty was [one that arose] when he went down [into the water] he is impure, but if the uncertainty was when he was coming up [out of the water] he is pure. Rabbi Yose says: even if there is room [in the water] only for one person plus the impurity, he is [still] pure.

סְפֵק טֻמְאָה צָפָה עַל פְּנֵי הַמַּיִם, בֵּין בַּכֵּלִים בֵּין בַּקַּרְקַע. רַבִּי שִׁמְעוֹן אוֹמֵר, בַּכֵּלִים, טָמֵא. וּבַקַּרְקַע, טָהוֹר. רַבִּי יְהוּדָה אוֹמֵר, סְפֵק יְרִידָתוֹ, טָמֵא. וּסְפֵק עֲלִיָּתוֹ, טָהוֹר. רַבִּי יוֹסֵי אוֹמֵר, אֲפִלּוּ אֵין שָׁם אֶלָּא מְלֹא אָדָם וְטֻמְאָה, טָהוֹר:

Mishnah Taharos 4:9: What is the case of the uncertainty of liquids regarding whether they themselves have become impure--and where such [liquids] are impure? If an impure person extended his foot amid pure liquids and it is uncertain if he touched them or not, its uncertainty is impure [i.e. the purity status of the waters is uncertain, and therefore impure]. If there was an impure loaf in his hand and he threw in among pure liquids and it is uncertain whether they touched or did not touch, its uncertainty is impure [i.e. the purity status of the waters is uncertain, and therefore impure]. And what is the case [of the uncertainty] regarding whether they [liquids] rendered [something else] impure--where such [items] are pure? If there was a stick in his hand on at its tip were impure liquids, and he threw it among pure loaves, and it is uncertain whether they touched or did not touch, its uncertainty is pure [i.e. the purity status of the loaves is uncertain, and therefore pure].

סְפֵק מַשְׁקִין לִטָּמֵא, טָמֵא. כֵּיצַד. טָמֵא שֶׁפָּשַׁט אֶת רַגְלוֹ לְבֵין מַשְׁקִין טְהוֹרִין, סָפֵק נָגַע סָפֵק לֹא נָגַע, סְפֵקוֹ טָמֵא. הָיְתָה כִכָּר טְמֵאָה בְּיָדוֹ וּזְרָקָהּ לְבֵין מַשְׁקִין טְהוֹרִין, סָפֵק נָגַע סָפֵק לֹא נָגַע, סְפֵקוֹ טָמֵא. וּלְטַמֵּא, טָהוֹר. כֵּיצַד. הָיְתָה מַקֵּל בְּיָדוֹ וּבְרֹאשָׁהּ מַשְׁקִין טְמֵאִין וּזְרָקָהּ לְבֵין כִּכָּרוֹת טְהוֹרִין, סָפֵק נָגַע סָפֵק לֹא נָגַע, סְפֵקוֹ טָהוֹר: