Here's a response from Wendy Greer, a Feng Shui coach at Learning Strategies:
The biggest concern would be lack of foundational grounding beneath the structure. This came up in one of Marie’s teleseminars when someone was renting a room above a garage.
Here were her specific recommendations in that circumstance:
What do you need to do if you have a bedroom or office over a garage?
In this case you would have an emptiness beneath you and are missing foundation and grounding. Make sure that the carpet or tile that you use has an earthy feel to it so it feels grounding. You could also add fire to enhance the earth. Keep the garage as clean and organized as possible (remember that energy radiates in all directions). Also be sure that you are not sitting or sleeping on top of the heater, washer, dryer, etc. which create an energetic, electro-magnetic disturbance. Use lots of plants and greens if an office or pictures of flowers and nature images in the bedroom.
The other concern would be a potential imbalance of the elements with that much water around them. I would suggest they use a fair bit of interior wood to weaken the water and earth to control it if they decide to go ahead with this project.
Much Love,
Shawn