First, let's summarize the Mishna and the Tosafos.

The Mishana cites three incidents where a non-Jew does Melacha on Shabbos. The central principle is that if the Melacha was done exclusively for himself, then the Jew can partake in any secondary benefits. However, if the Melacha was done with the Jew's benefit in mind, then the Jew may not receive any benefit from the act. The Gemora asks why we need three cases to demonstrate this point, and answers that if we had just brought the case of the candle, I would have thought that that the water case was prohibited because (unlike the candle case where one candle is enough for many people) in the water case we might have been concerned that at another opportunity the Goy would draw and carry water intentionally for the Jew and therefore the Rabbis would have prohibited this. We be therefore brought the case of water to show that this is not so and the Jew may enjoy secondary benefit. Please note, that the Mishna added the case of water to PERMIT benefit.

Tosafos points out that the case of "water" is set out in the context of the Goy drawing water for his animal and NOT for himself to drink, and consequently the Issur when the Goy draws for the Jew is ONLY when he draws water for the Jewish ANIMAL and not if he drew for the Jewish MAN himself to drink. The rationale for this is that since the Jew himself could enter the cistern to drink without Chillul Shabbos, it is not considered "obtaining pleasure from Chillul Shabbos" to drink that water. Tosafos is bothered, (seemingly) by the problem that the whole point of the Mishna is to PERMIT the Jew to drink when the Goy drew water for himself so why are we saying that there is a Chiddush that there is an PROHABITION when drawn for the animal (with that being a proper Chidush in its own right as explained above). On that Tosafos says the we could have used the case of a man drinking to learn the extension of the Heter of the candle to the drinking of water, but since we set the Din in the context of an animal drinking we learn the Heter of a man AND ALSO the PROHABITION of the animal, and there is no question on the Tana.

I hope that clarifies your question.

Shimon Brodie