The rate of water evaporation is proportional to the absolute difference between the saturation vapor pressure of water in air at pool water temperature and the actual vapor pressure of water in air at air temperature. The relative humidity percentage is the ratio of the actual vapor pressure to the saturation vapor pressure. So if the air and water temperature were the same, then at 100% relative humidity there will be no evaporation. This should make perfect sense logically since 100% relative humidity means that the air cannot hold any more water, so how could any water evaporate into it?
The rate of water evaporation is also dependent on temperature with higher temperatures resulting in faster water evaporation. This is further accelerated when the water temperature is higher than the air temperature -- basically, the water temperature is related to the saturation vapor pressure (for the formula) while the air temperature is related to the actual vapor pressure.
Finally, the rate of water evaporation is GREATLY accelerated by wind, especially near the surface of the water. A 5 MPH wind at the surface of the water (so higher wind further above the water) results in roughly 3 times the rate of evaporation.
There is a myth that the evaporation is very different between the day and the night. In fact, the vapor pressure of water in the air doesn't change a lot between day and night because as air temperature cools (so the saturation vapor pressure is lower since the air can hold less water), the relative humidity rises. The net result is roughly the same absolute amount of water in the air (there is a little less at night after dew is deposited). The saturation vapor pressure at water temperature doesn't change so the amount of water evaporation day and night is roughly the same. Of course, you may see more of the evaporation at night because the evaporated water cools as it mixes with the air and then becomes visible (i.e. small water droplets are formed as the cooled gaseous water liquifies when the air is saturated with water locally), but evaporation is happening during the day as well except that you don't see it because the water remains in a gaseous state.
So there are really only two questions you need to answer to know if there is a lot of water evaporation over the winter. 1) Does the air have more absolute amount of water in it and 2) Is the water temperature still warm. If you stop heating the pool and let the water cool during the winter, then the rate of water evaporation will drop considerably. As for the first question, let me take my own city of San Rafael, CA as an example where I will compare the air temperature and humidity in July with that of January.
Jul Day -- Temp 74.5F; Rel. Humidity 49%; 10.72 mm Hg vapor pressure
Jul Night -- Temp 51.5F; Rel. Humidity 85%; 8.28 mm Hg vapor pressure
Jul Avg. -- Temp 61.9F; Rel. Humidity 68%; 9.64 mm Hg vapor pressure
Jan Day -- Temp. 58.4F; Rel. Humidity 43%; 5.38 mm Hg vapor pressure
Jan Night -- Temp. 37.2F; Rel. Humidity 77%; 4.34 mm Hg vapor pressure
Jan Avg. -- Temp. 47.1F; Rel. Humidity 60%; 4.96 mm Hg vapor pressure
Let's say that a pool is heated in July to 88F. With no wind, the rate of evaporation during a July day would be 0.28" per day and during a July night it would be 0.31" per day. If the pool were still heated in January (unlikely), then the evaporation average for day/night would be 0.35" per day (24 hours). However, if the pool were at the average temp in January of 47.1F, then evaporation would only be 0.04" per day.
So, bottom line, is that if the pool water is unheated and gets cool, then evaporation over the winter becomes very low -- assuming no wind.
As for a pool cover, if it is solid (not mesh), then evaporation is pretty much stopped since a small amount of water evaporates to saturate the air under the cover and that's all. So if you lost any water, then you've got a leak.
As for central Massachusetts, this map show an annual evaporation of unheated water in Massachusetts of around 35" per year (about 0.1" per day). If I look at average temperatures in Worcester here, it gets cool when you have the pool closed so evaporation would be lower. Also note that there is rain so I would think that would make up for a lot of the evaporation during the winter if there was no cover.