Unless your taking temp from pressurized water it will ALWAYS register lower than the air temp. Water expands(though very little) as it becomes warmer, this is heat to energy conversion. You will usually lose 1 to 2 degrees using a water wiggler or any other device using water. If the water is in a container and open to evaporation it will lose even more temp.
I use sand in a plastic Easter egg for taking my temps, the sand is a solid and it will absorb the heat for a stable temp, though it may rise or fall 1 degree depending on the type of thermostat you are using. For a more stable temp you can use a larger container(pill bottle) with the sand.
The water wiggler is fine to use as long as you take into account the temp loss due to expansion. If your temp stays stable use a oral medical thermometer(very accurate), to calibrate. But remember that the thermometer only measures the top temp as it will not read lower until it is shaken back down. This is good too as it will not drop when you remove it to read it. When using it make sure to use it at a location which would be the middle of an egg. Or you can put a egg sized pill bottle of sand, let it stabilize, and then measure the temp with the medical thermometer.