If Jacques were to hold his breath the air in his lungs would be kept at a constant temperature. Would it be safe for Jacques to hold his breath while ascending from the bottom of the lake to the surface?

It isn't safe at all ...

Explanation:

Even if remains at constant temperature, while ascending to the surface of the lake Jacques lung pressure will increase if he hold its breath. Thats because the pressure of water weakens and the pre-hold breath in lungs will expand as he rises. This can cause serious lung damage and drowning is possible.