JoeBloggs2 Technology has moved on a little bit in the last 20+ years...
True for signal processing and batteries, but AUVs that can run a long distance (greater than 100km at a stretch) side scan sonar profiling run don't exist currently, at least
afaik.
Constructing and then co-ordinating the fleet, keeping them on course to ensure gapless coverage, recovering them reliably to recharge/refuel them in challenging weather and then integrating the results is a novel task, one that would take years to do.
It would be quicker and cheaper to use chartered oil survey vessels, they exist in quantity, their operating costs are well known and they have the needed skills.