1. The Gemara discusses whether one may pour water into the sea through the hole in the platform (see Daf 87, #4).
2. On Shabbos one may not pour water into a Chatzer that is less than four by four Amos.
3. One may do so (#2) if he has made a drain in the Chatzer that is large enough to hold two Se'ah of water.
4. Chananyah maintains that one may not pour water onto a roof, even if it is 100 Amos wide.
5. Abaye says that if a person made a drain, he may pour into it as much water as he wants on Shabbos.
A BIT MORE
1. There are two versions of a statement of Rabah bar Rav Huna. In one version he says that it is forbidden, and in the other he says it is permitted.
2. This is prohibited because the water is clearly going to flow out into the public domain.
3. He may make it either in the Chatzer or outside. If he makes it outside of the Chatzer, he must ensure that it is a Makom Petur.
4. This is because the water runs off the roof, as opposed to being absorbed into the surface of the roof.
5. Abaye permits this even if the amount of water poured is more than the amount of water that the drain can hold. Once the person has made the drain, its presence reminds him not to pour water directly into the public domain, and therefore he may pour as much as he wants into the drain.