For a real defense, I would recommend a high wall, not so much for pirates, there isn't really very much of that but for rogue waves. The wall would be expensive if it didn't have other uses such as a store or other business or even apartment homes. Make everything pay for itself through double duty.
The South Seas are very remote and not in much danger of marauders while Marinea is close to other countries that pirates would want to stay away from. In addition, Marinea has the ability to grow larger than many island nations because it can expand with demand, not easily done with islands made of dirt.
All it takes to join Marinea is to buy a boat, about what you'd expect to pay for a house, and drop anchor close to where the others are moored. Marinea could well have more than a million residents, what pirate would want to attack something that big.
A much bigger problem is the danger of nuclear war. We can start an ocean community with as little as $15 million but the ability to survive a war will require an ability to go under water and come back up when the danger passes. Same for big waves. But we will have to design that after we get on site. We need to get started.
The biggest problem facing ocean colonization is fear and perception. If people don't believe something is possible, they won't try. Why isn't everyone here members of Marinea? Because they just don't believe it will work. Until we learn how to overcome the negativity in beliefs, we will continue to struggle. We saw this a couple of years ago and decided to create a pilot project to prove the feasibility of seasteading. In fact, that is where Marinea came from. Phase one is proof of concept. We need $15 Million for that. Put another way, we need 150,000 members. Once we reach that goal, seasteading will be a fact of life and everyone will be moving in that direction.