According to Official Guinness Records,
The most electricity generated by pedalling underwater is 2,502.2 watt hours and was achieved by Lloyd Matthew Godson (Australia) at Legoland Atlantis by Sea Life in Legoland® Deutschland, in Günzburg, Germany, on 13 April 2010.
Lloyd Matthew Godson spent 14 days in a small habitat on the ground of an aquarium starting from 12 pm on 30 March 2010 to 12 pm 13 April 2010.
For a complete list of 2010 records, please visit 2010 Guinness Records in Germany.