Sorry but this isn't strictly correct. When plants photosynthesize they produce oxygen. In submersed plants most of the time this oxygen dissolves into the water completely unseen. The plants are still producing oxygen and photosynthesizing and adding that oxygen to the water.
What it seems you are seeing is a process called pearling. Pearling occurs when the layer of water directly around the leaf or leaves is saturated with dissolved oxygen and therefore no more can dissolve into the water. Any more oxygen produced then collects as bubbles of gas which then rise to the surface when large enough.
When, or indeed whether, pearling occurs is dependant on a number of factors including the rate of photosynthesis (which is governed by a raft of factors such as the levels of CO2, light and fertilisation), water circulation and the level of oxygen in the water.