Can't answer exactly.... though I bet some google searches might give the avg temp by month. When I was there water skiing in mid-summer, the water was fine for swimming.
The Southern End of Willoughby is one of the most scenic spots in all of New England IMO.
Maybe you know this and are asking about going 'again' this time just to swim. But if you've never been go regardless of water temp. if you have any interest in a scenic spot.
I've been there in July several times and always seen people swimming. But I was only fishing not swimming so can't say how warm the water actually was.
Please suggest me a lodging at the Southern End of Willoughby.
I don't believe there is any other than a campground and perhaps some of private cottages are being rented. But try searching as I might be forgetting something.
Willoughvale Inn, at the other end, is nice.
My guess is that the average temp in the top 4-5 feet of water is in the low 70s by early July but that will fluctuate depending on recent weather. Willoughby is a very deep lake for its size, so if there's wind and rain, and things get churned up, it can bring colder water up to the surface. On the flip side, several warm days with light winds can get the top layer of water pretty toasty. By early next week, with the current heat wave upon us and not much wind in the forecast, I wouldn't be surprised if some temp readings near the surface get into the 80s. It's going to be a situation where you swim along and feel significant temperature changes in the water, between the colder water from below mixing with the sun-warmed water at the surface. In any event, it looks like for the foreseeable future, a dip into any water source in VT will be a refreshing break from the sweltering heat.