The tree would die, it needs a combo of air, water, and sun to live.

Interesting sidenote to this tho, there is a hydroponic system called “kratky” where the plant roots go directly into a jar of water. As the plant grows the water level goes down, exposing more of the roots to oxygen, which is necessary for the plant to live/grow.

It’ll die, the leaves would turn into a slushie. The trunk would keep it’s appearance underwater (logs are sometimes kept under water for processing.) And your whole aquarium would turn green from algae and mold pretty quick and you can’t look inside anymore.

Your question isn’t stupid, but it does make me feel very curious about what potential hypotheses you might have been weighing.