Why be a pessimist, when the mechanical problem was solved some 60 years ago (self-winding wristwatches) and updated more than a decade ago with Seiko's "Kinetic" series, which use a small dynamo to generate electricity to power their watches the same way?
Bobbing, rocking, rolling, all could easily turn a dynamo and generate electricity. How much motion, how much bulk, how much cost, how much output, these are all trivial variables compared to the larger question of a tiny niche market.
There are much larger projects to use ocean tidal/wave motions to turn dynamos in a similar way. of course, they're on a much larger scale and unlike boats, those locations are chosen to be in motion rather than at calm rest.
Yes maybe you're right but the fact is that a kinetic watch uses absolutely minute amounts of electricity. Multiply the generated power by 1000 and that technology will probably provide enough power to run one LED but you have a device that won't fit into your boat. And Seiko Kinetic holds as minute a portion of the watch market - if they were that good we'd all be wearing one.
Also, the first generator that was designed to use wave action was done over a century ago and still very few have lasted or keep working anywhere in the world. The list of inventions is long, the money poured into it is immense, the list of successful installations is pitiful.
And then of course I believe the point of my post went by you - I make a considerable science out of finding places to anchor were my boat will not bob, rock or roll. The harder I try and the better I get at it, the less any such device will deliver. I can't see any sailor choosing a rolly anchorage just get back 10 amp hours over a night. It's a contradiction in terms.
And finally, my pessimism lies in the fact that energy world wide is a trillion dollar industry and hundreds of millions of dollars of R&D have not yielded a system like this and now someone is developing one in their garage? Especially for yachts?? Yes of course they are. I can't wait.