Yea. Its super easy. We built a water coliseum same way.
The trick is to find a decently shallow portion of water or ocean, doesn't even have to be flat ground under water. Then cover the floor of the ocean with connecting pillar/ceilings. Make it cover basically the whole area you want for your base. Then run one pillar all the way up to surface and you can build the entire ground of the base with ceilings off that one pillar.