Put up a tent in the various places and do a little camping to get a feel for what is "right".
Some thoughts: On the west side, you will get morning reflections from the water, on the east, sunset. Do you prefer extra light in the morning or evening?
Humidity on construction, you will get more too close to the water. Consider that.
My personal opinion is somewhere in the range of having some grass in the front for a buffer and area to walk around, so maybe 50' or so.
