I've posted this before but since this thread is about water temperatures I'll mention it again. Water temperatures are generally taken at the surface or a few feet down. Your depth finder / fish finder is the same. This time of year through mid to late June there is a thermocline in the Western Basin. As spring advances and the water warms, it is the surface layer that warms first. It might be 42 degrees at or near the surface but the bottom, where the walleye are spawning, might still be 38 degrees. This is especially true out off the reefs in the deeper water. The shallow reefs do warm more quickly and winds / currents mix the water there faster, but the thermocline is usually still present. If you really want to know the water temperature where the walleye are get and take along a water temperature thermometer. You can purchase them pretty cheap, from simple standard mercury tube to fancy electronic. Once you get out to the area you expect to fish drop it over and take the temperature just off the bottom. If you find an area with a little warmer bottom temp that's probably a better place to fish, everything else being equal or similar.