its helpful to an extent... mine is hooked to boat, about a foot below water for the "surface temp" but much more than that your wasting your time...find the thermocline on your graph, and fish above it.
Even though I have an integrated SIMRAD NSE system with the most advanced transducer/sonar/sounder available, I still deploy the Fish Hawk X4 and the Fish Hawk TD when walleye fishing.
Sure I can usually see the thermocline on my SIMRAD NSE12 unit's sonar but the X4 provides the actual temps at both surface and depth (as well as relative speed at both). I will use the TD for the few times that I can't see thermocline on sonar as it will give me temp readings every 5' of depth - i just put on walleye rod with heavy weight and drop over the side to bottom and then retrieve - its quick and you don't have to worry about water depth as it comes up with a depth and temp paired on the display. It is interesting with the TD to see the actual temps where the fish are hanging out versus where they are feeding. In heat of summer the temps will vary by 10+degrees in a span of 5'.