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: