Stephen Cox

Thanks for those ideas; have decided to place fixed magnet alternator inside an old sump pump housing. Those things operate submerged for long periods, so the housing arrangement must be reliably waterproof at depths of a meter or so. The alternator will not be submerged (it will be on the back of the raft), just might get temporarily swamped.Now to find a second hand sump pump with sufficient internal dimensions for my generator..

Hi Andrew, no answers here, but shared desire for an underwater generator.
Have you seen idenergie (canadian). http://idenergie.ca/en/features/ They must have solved the issue.
I am building a raft turbine, which will sometimes submerge, to make 100w. Plenty of generators are available from china e.g. http://www.ten-high.com/productsinfo.aspx?productid=197#.WnGY_0x2udt which I am buying.
I'm going to find a waterproof housing (a box), and run the shaft into the housing via 1 or 2 marine bearings. Not quite researched that bit yet.
I'm looking at http://www.directindustry.com/prod/thb-bearings/product-57112-597289.html or something similar.

Here's a link to my raft (so far): https://photos.app.goo.gl/ZbuyLEK9pc25nN4D2

Best wishes,
Steve