Final answer:
The minimum theoretical concentration of excretory fluid needed to have a net gain of H2O after drinking seawater depends on various factors, but it would be less than the concentration of salt in seawater, which is about 3.5%.
Step-by-step explanation:
The minimum theoretical concentration of excretory fluid needed to have a net gain of H2O after drinking seawater depends on various factors. When a person drinks seawater, the body tries to eliminate the excess salt by excreting it in urine. However, this process requires a significant amount of water. So, in order to have a net gain of water, the concentration of excretory fluid needs to be lower than the concentration of salt in seawater. This means that the minimum theoretical concentration of excretory fluid needed would be less than the concentration of salt in seawater, which is about 3.5%.