Sonar doesn't typically work very well looking forward near the surface. For larger craft the transducer could be mounted low enough to look up slightly but ev

Sonar doesn't typically work very well looking forward near the surface. For larger craft the transducer could be mounted low enough to look up slightly but even then you have surface interference issues. A container might be a big and deep enough target to pick up on something mounted to the bottom of a sailboat keel perhaps but definitely not the bow. From a signals perspective, things that live right on the edge of the water/air boundary are kind of a pain to detect. Unless the water is perfectly still, there's so much noise that you need a lot of spatial resolution to actually see anything useful