This is tougher than you might think. GPS does not work underwater. Inertial navigation is also questionable. Knowing exactly where you are to make a map is a difficult task.
From the Article:
“Because GPS is not available underwater, most underwater robots do not have an absolute position reference, and the accuracy of their navigation solution varies,” explains Brendan Englot, an associate professor of mechanical engineering at the Stevens Institute of Technology, in Hoboken, N.J., who was involved in the study. “Predicting how it will vary as a robot explores uncharted territory will permit an autonomous underwater vehicle to build the most accurate map possible under these challenging circumstances.”
This is an unusual and interesting problem set. Enjoy!