Thermals form downwind of a lake. This can happen if that area of the air mass is stratified. The cool surface lake air is channeled around an area of moderatley warm surface air, and as that surface air ascends into the cooler air well above the surface, it ascends rapidly in the form of a thermal. Such a thermal will be a cooler thermal than those not downwind of a lake. That is, it wouldn't have formed except for the localized area of cool air.
Thermals form as the surface air is cooled due to a sea breeze, forcing the warm air aloft.
As the sea breeze moves inland, so does the line of lift. As the line of cool air moves inland, the pockets of warm air release into the cool air that had just passed above. Thermals form due to a convergence of cool air and warm air. Don't be intimidated by the adiabat chart. If you can learn to fly a glider, it won't take you more than a few minutes to learn how to use this effectively.