A moteur for hydrogenerator?

[MichaelGH]

I am a student in a university in France. I have to do a project this sem that involves making a hydrogenerator. However I have no idea how to choose the moteur that will generate the electricity. The only thing I know is that the battery on the boat of our client is 100mA and used up in 25 hours. And that the boat usually sails at 5-7 knots. What are the criteria necessary to choose the right moteur?