To what does Pi attribute his survivng the fall from the ship to the water in Life of Pi?
Your question is confusing because Pi survives the fall from the Tsimtsum into the lifeboat (due to a "bounce on the half-unrolled tarpaulin"), but the lifeboat is still hanging in midair at that time. Pi also survives the fall from the lifeboat (after the zebra jumps in) into the water mostly likely due to the water, itself. Then finally, Pi only manages to make it back to the lifeboat from the water because of the "lifebouy."
Let's look at each in turn. The men on the boat throw Pi (against his will) onto the life boat where "I landed with a trampoline-like bounce on the half-unrolled tarpaulin covering a lifeboat forty feet below." Forty feet onto a sheet of canvas half-covering the small boat. It would have been a fatal fall! Therefore we can attribute this first element of survival to a canvas sheet stretched across half of the small life boat. Ironically, it isn't clear from the novel whether Pi lands in the life boat or in the water. I assume he landed in the boat itself because, later, he says "we hit the seething water."
Secondly, we can attribute Pi's survival to the water itself. At this point, the zebra jumps into the boat. The zebra isn't lucky enough to hit the tarpaulin. Weighing more than five hundred pounds "it landed with a loud crash on the last bench, smashing it and shaking the whole lifeboat ... The lifeboat fell through the air and we hit the seething water." Obviously, the surface of the water is a lot more forgiving than a surface such as cement; therefore, Pi survives.
Finally, Pi's ultimate survival depends on a lifebuoy. When Pi is confronted with the water, he is terrified: "It was black and cold and in a rage." He would not have survived and would have drowned right there. As he searches for something to grab onto, he first finds an oar, but it isn't enough to keep him afloat. "If there hadn't been the lifebuoy I wouldn't have lasted a minute."
Thus, you have to clarify your question in your own mind to figure out the right answer. Be it because of the tarpaulin, the forgiving surface of the water, or the lifebuoy, Pi Patel DOES survive.