Is it possible that new marinas can be built slightly off the coast?

The question was in the title. Of course it's possible. Just like airports. In some places they do that. It provides some technical challenges, like the multi billion project sinking into the silt, but it can be done. Mostly in places where land is 500,000 an acre. Most of the swampland called the east coast, not so much. MAYBE in busy areas where one single gate can generate millions of dollars a week. Doubt you'd want to pay that for your slip.

Dubai and China are modern examples of island building. My question is why? It might be a thrill for megayachts, but the extra cost in keeping my tub there would drain my pockets I suspect. The infrastructure construction and maintenance costs would have to be astronomical?

Off the coast, No. The surge is incredible, even if there are breakwalls. Then, what about the electricity? Yachts use lots of it. Then the EPA and DEP would never allow it in the U.S. It has been done here back in the day, Fisher Island in Miami comes to mind. Then the other issue, is people want convenience. They go to work on their boat and forget 1 tool and have to do a 30-60 minute round trip dinghy ride? I don't think it would fly for a lot of people.

You answered your own question there.

https://en.wikipedia.org/wiki/Wave_power

20 years ago I thought about a under water marina, slightly offshore in a bay, Technically doable, not sure about financially. I'd go for a robotic bottom washer in south Florida where bottoms need cleaned monthly to quarterly at most. Get a CNC gantry and turn it upside down, design it to be non polluting then sign everyone up. Essentially a car wash for boat bottoms. Local Divers might not be so happy.

I luv your enthusiasm, but robotic washers are the least of the challenges in my opinion.. However this might not be a bad investment if the market is investigated and long term capital can be attracted. Leveraged, the initials are looking to make the money

Easy, marina's are surrounded by land and buildings on 3 sides usually which knocks down the wind. They're usually not in open water where you can get large waves from hurricane force winds. If someone wanted to tie their boat up away from land, why wouldn't they just anchor it for free?

Free? Free?? Not after the revenuers find you.

I don't know how it works in the US, I have the impression that multiple levels of government have jurisdiction over the use of water. In Canada, it certainly wouldn't work, all waters are "owned" by the Federal Gov't and they'd be all over you if you tried to build something without jumping through exhaustive paperwork hoops.

I’ve seen plans of something similar a developer is planning for Maule Lake at Biscayne and 172nd St. in North Miami Beach. Residents of Eastern Shores are taking it seriously.

lol ...no kidding.

This concept was tried in Maryland some years ago. It did not work out.