My 2cents. Your thermometer at the dock 20" deep is okay for a overall pond water temperature. However if you really want to check the water temperature where most of the eggs are being laid you need an indoor-outdoor thermometer with a probe attached. Most of these units have 10 ft long probes. Check / calibrate the temperatures of the dock thermometer and the in-out door thermometer to verify or determine the readings at 40, 45, 50 and 55F. Use ice in water to get the test water to your needed standard temps. We have to assume one of the thermometers is accurate or our standard. I have a quality mercury thermometer that I use as my standard.
When you know the variation of the thermometers, put the probe weighted with a fishing sinker on the spawn area bottom in late afternoon the measure the bottom water temp. Adjust the reading to match the calibration temp difference. Bottom water is the area the YP are sensing temperature differences along the shoreline. Overnight & during the day the convection currents will mix all this water to a pretty uniform temp until the afternoon sun again heats the bottom in the main spawning area.
Example of a decent digital thermometer with 10ft probe http://www.ebay.com/itm/Taylor-Precision-Products-Digital-Indoor-Outdoor-Thermometer-1522-/381901088656?hash=item58eb142790:g:7WUAAOSwKOJYIm-v
