If a plant were under water and was photosynthesizing, what gas would be visibly bubbling from the plant?

Yes gas can be seen in bubbling if a plant performs photosyntesis under water.

Explanation:

First of all to understand this answer we need to remember the process of photosynthesis. Photosynthesis is the process a plant carries on to create energy through the chloroplasts. While the plant does it, it uses gas to breathe and obtain energy from light and the air. So, because plants only need carbon dioxide to perform their activities they expel oxygen. That is why they release gas because they also breathe and because oxygen is expelled from their tissue.

the answer to your question is sperm