I doubt it. Compared to a fixed tower wind turbine you have hull drag losses, water turbine losses, and the problem of where to put the power since you won't have cables to shore.<p>Some cruising sailors do use systems like this to generate their own power when under way.
'useful' is subjective. That mast would not have to be 'very large' to power one's GPS system, and having that certainly could be useful. At even lower power, a tiny impeller that measures boat speed could be very useful.<p>If you are aiming for large amounts of power, I would expect it to be more efficient to replace part of the sailing by a wind turbine, and to get rid of that water turbine.
Take a look at this; I submitted it a couple days ago and found it very interesting:<p><a href="http://thepowergeneration.blogspot.com/2009/09/hydrogen-challenger.html" rel="nofollow">http://thepowergeneration.blogspot.com/2009/09/hydrogen-chal...</a><p>If you had other ships carrying away the generated hydrogen, it seems like a brilliant idea.
My friend Craig, an engineer for an oceanography company, has been working on very similar ideas for a while. Here are two of his proposals:<p><a href="http://longshotcity.com/2008/11/17/flymill/" rel="nofollow">http://longshotcity.com/2008/11/17/flymill/</a><p><a href="http://www.paratow.com/" rel="nofollow">http://www.paratow.com/</a>