The Watch Leader is the member of the Corps of Exploration who is in charge during each 4-hour watch. They make the decisions on what the team will be doing during each watch, decisions that can include what routes the ROVs will take on a dive, what samples to collect, which areas need special data collected, and any other moment-to-moment mission specific requirements. The Watch Leader works closely with the Lead Scientist to understand what scientific needs are priorities during each dive.
Watch Leaders often have advanced degrees in a field of science related to the expedition, such as biology, geology, or archaeology. Other times they are graduate students in one of those disciplines.