Hollow, more or less. Hoses, lines, cables, etc. run through them. But they are designed to vent the water from the top deck, back to the ocean.
I believe that is what you can see on one of the legs. A large grilled vent.
The grill doesn’t look low enough in the structure to drain all of the water. It may be more for access and heat and/or gas escape.
It would appear that some of the concern is over the salt in the water as well. I wonder why? I would surmise that salt water is heavier than foam, but not sure.