A few points I've thought of are collideing with the whales, if they are made as an actual physical object perhaps they could be given 'enemy merchant AI' to make them 'run away' from your boat.
Also what about your hydrophone operator? Will he call it out as a merchant contact, and send you on a wild goose chase after this invisable merchant? Or will it be unreported and you have to find it yourself?
Just trying to point out some things that might need to be thought out before starting
It sounds like a promising project, I'm always up for things that add atmosphere
