The tree would drown.
Trees require evaporation through the leaves to draw nutrient enriched water from the roots through the trunk to the leaves. Which then evaporates and causes the process to be continued.
They also require Carbon for the photosynthesis process, which trees absorb via the leaves in the form of Carbon Dioxide.
What would happen then if you took a tree which only has leaves at the top and submerged it to just under the leaves? Does the bark need to be exposed to air as well or would it just rot out like timber does?
Mangrove trees live submerged in saltwater for example. This video also goes over some of the adaptations it has to do that (ex. Special roots to allow it to stand upright in soft unstable ground)
Basically the same way, but instead of absorbing gasses through the air, it's through the water. They don't have stomata (air holes) that are found on plant leaves that exist on land and I believe they can absorb carbon directly through their cell walls.