I'm not a scientist, so I don't have any specific information about the impact of marine mammals on ocean productivity. My gut instinct is that they probably serve a pretty valuable function to the ecosystem, but that's really based on nothing and shouldn't be taken seriously.

I do think that sound being invisible and underwater is a big part of the issue. And beyond that, you have to get people to understand the impacts that underwater noise has on marine animals. Honestly, I think maybe the way to go on marketing would be something more anthropocentric: getting people thinking about how unpleasant it is to live next to a construction site or an airport might be a good entry point into the issue. 

This is a great write-up, thank you for sharing it. Overall, I think I share your skepticism about this as a viable cause area. I'm new to (but very interested in) WAW, and I think it's very interesting to view this issue from that perspective. I've come across the issue in my work (confidentiality precludes me from going into many details) and it looks like your understanding of the issue is spot-on. 

A few general points:

• Tractability strikes me as a major concern here. In my experience, underwater noise tends to be an issue that's heavily entangled in industry, various regulatory regimes (possibly from many jurisdictions), and other local circumstances.  Countries are also more touchy about marine boundary issues than you would think at first blush. I think it could be very difficult to pinpoint where advocacy should focus.
• Don't underestimate the value of marine mammals from a movement-building/PR perspective. Most people (and governments) view fish and invertebrates as resources open to exploitation rather than moral patients. I don't agree with this, but it tends to be the prevailing sentiment among regulators and local communities (in my experience). There are other animals (orcas and dolphins come to mind) that people care a lot about as moral patients and around which advocacy can be built. 
• Underwater noise can be a difficult issue for newcomers to immediately embrace as important. It's not necessarily intuitive why it matters. I think there could be a lot of value in figuring out how to package the issue so that it can be understood (at least roughly) through an advertising campaign in regions where it is politically salient.