Final answer:
Jack Sparrow and Will Turner's underwater survivability with a rowboat over their heads depends on air volume, oxygen consumption, and water temperature. Hypothermia is also a risk factor. Their survival time could realistically range from minutes to over an hour under ideal conditions.
Step-by-step explanation:
The scenario described in the question is not scientifically detailed in Pirates of the Caribbean I, so a precise answer about how long Jack Sparrow and Will Turner could survive underwater with a rowboat over their heads is impossible without additional information.
However, the survivability would depend on the air volume trapped under the boat, the rate of oxygen consumption by Jack Sparrow and Will Turner, and the diffusion of carbon dioxide out of the enclosed space.
The larger the volume of air trapped and the more efficient the gas exchange with the surrounding water, the longer they could survive.
Another consideration is the water temperature, since hypothermia could set in if the water is cold, leading to a decreased survival time.
Survival in these conditions also depends on the physical condition and calmness of the individuals, as panic can increase oxygen consumption.
The fictional characters seemed to defy many realistic physical challenges with clever tricks, but in reality, the survival time could vary from a few minutes to perhaps an hour or more under very favorable conditions.
This estimation assumes that both characters are average adults in terms of oxygen consumption and that the trapped air is not replenished.