Which of the following factors does NOT determine swell size?

Swell size is influenced primarily by wind-generated factors such as the fetch of wind, velocity of wind, and duration of wind. The fetch refers to the distance over water that the wind blows, which directly impacts how large wave systems can develop. A longer fetch allows waves to build up more significantly. The velocity of wind is another crucial factor; stronger winds can generate larger and more powerful waves. Additionally, the duration of time that the wind blows consistently over the water also contributes to swell size; longer durations lead to larger swells due to increased energy transfer to the water.

In contrast, the temperature of water is not a direct factor impacting swell size. While warmer water can have various effects on oceanic and atmospheric conditions, it does not influence the initial size of the waves created by wind. Thus, it is the only choice that does not fit as a determining factor of swell size.