or more conventionally, you build a fleet of oceanographic survey ships which operate in the open navigating international waters, collecting vast data sets of bathymetric information... obviously not perfect yet.

https://en.wikipedia.org/wiki/Pathfinder-class_survey_ship

there is also the danger that a drone emitting sonar pings may be indistinguishable to a third party from a live torpedo, and makes it look like you're preparing to attack somebody you really don't want to start a war with (China, etc).