Can the clearway be located on a lake?
From USA AIP (emphasis added):


  The U.S. does not require or provide criteria for clearways in its design standards. It does encourage ownership and clearing of the land underlying the innermost portion of the approach out to where the approach surface is 10.5 meters above the level of the take-off surface.


You may also want to check AC 150/5300-13A - Airport Design (no mention of water).



FWIW, ICAO includes water in the definition (Annex 14 to SARPs):


  Clearway. A defined rectangular area on the ground or water under the control of the appropriate authority, selected or prepared as a suitable area over which an aeroplane may make a portion of its initial climb to a specified height.