Could be an otter, however there is literally nothing to compare it against to determine the size of it. the clear wake after it would suggest it is actively moving to the right, so is unlikely to be driftwood or an inanimate object since there is no tidal movement in a lake. It could be an eel, I heard that sterile eels can grow phenomenally broad.
Basically its a crap photo that neither confirms nor disproves anything, perfect for speculation. My theory is its Brian Blessed out for a morning swim.