With a caisson, anything that is weldable is field-maintainable these days. At least, carbon steel is, I have on good authority.

Concrete I am deeply conflicted about. Wire-reinforced concrete is the traditional material for oversized, under-capitalised "heartbreaker" projects that end marriages but never leave the building yard (except eventually to a landfill) :-p

Concrete has very good compressive strength and creep resistance. But is hopeless in tension and shear. The steel wire mesh upgrades "hopeless" to merely "poor". Until it rusts into powder and bursts the surrounding concrete as well. And it will - high pH concrete doesn't protect against chloride for long. Instead of steel reinforcement you might think to try using glass. But the high pH makes glass hydrolyse into gel. You can use stabilised glass with a high % of non-silica glasses, but there's no real advantage (cost or otherwise) over glass reinforced plastic then.

What concrete CAN do is something unique, and Luke I think you alluded to it with your "reef friendly" comment.

Biorock. Aka Seacrete.

If fully submerged in seawater, and if the reinforcing mesh is electrically conductive (metal or carbon fibre), and if a current is impressed on it (with the mesh as the negatively charged terminal), the concrete "grows" and "self-heals" with minerals from the seawater (mostly Brucite). Hard-shelled marine invertebrates colonise the surface and grow at amazing rates, using the electrical current to fix shell-building minerals more efficiently than they can in nature.

At least, that is what the patents and many articles tell us.

I plan on doing some experiments myself, starting this weekend.

For a permanently submerged module (or entire seastead), concrete could be amazing. I planned to do a proper writeup with links, but for now:

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