what are these?

Protective domes to cover the satellite antennae contained within.

Sometimes one of them might be empty and just installed for visual "balance."

The bigger ones are usually for TV when you are in the Southern hemisphere since most TV-satellites are in the Northern. The smaller domes are mainly for communication, like phone and internet.

Do they need to be protected??? Or are the covers more for aesthetics?

First protection. Aesthetics when you see them painted to match a superstructure or arch color scheme. Google around for prices (satellite marine communications) and your question of "are they expensive" and "are they air tight" should be answered.

Here's what's inside:

"Do they need to be protected?"

What PropBet said ...

They are very lightly built so that the tracking mechanism can move them rapidly to maintain the "view" of the satellite as the boat rolls, yaws, and pitches. If they were built to withstand strong winds and salt spray the inertia of the antenna would be more than an affordable tracking system could handle mechanically.

I was under the impression that the covers were stationary and only the dish inside rotated...

The antenna and tracking mechanism need to be protected because they're lightly built, they move inside the stationary domed cover.