Let's see Less. Stall speed for the Lake Aircraft I could find are 49 kts (56mph) with flap and gear down, 55kts (63 mph) with the plane clean. So the lake amphibian would have to be going at least 56 mph to take off and fly*. Flying at this speed for any length of time close to the ground or water would be hazardous, so typically once the plane is airborne it would accelerate to its best climbing speed, which is probably closer to 90 mph. My point here is that humans are very poor judges of speed, that why cops need radar.
*ground effect would lower this speed close to the ground, but the plane would still have to accelerate above this speed to climb.
It think we have a lake pilot or two on this forum who could validate or correct my numbers.
http://www.teamlake.com/PERFORMANCESPECIFICATIONS.htm