> Sea water is incredibly powerful and corrosive.
I'd point you to the Maunsell Sea Forts. Every single one has survived ~70 years in the open ocean, the ones that haven't were plowed into by boats.
My suggestion is specifically to be a giant sea fort, and to build inside of the column and above it. My idea would be to go ridiculously large and create a second interior wall after sinking as a sort of levy from the true interior in case the exterior is damaged.
I think a good 100 year zero-maintenance survival of an ocean arcology is amazing. Beyond initial construction the idea would be to expand and build a harbor, which would further expand the lifetime by taking any mechanical wear off of the main structure and place it onto the harbor walls.
Furthermore, if you had a crane on one of the towers (which would be mandatory if you're building as you go) it would be a simple matter to move soil around in the water to create an artificial reef.
The real problem would come with what materials to use for construction of the rest of the arcology. If you go with steel, it means you're going to require a lot of maintenance because of the sea air. It has longevity, (think all oil rigs) however it won't last forever. However, if you go with aluminum it will genuinely last for ever. The oxidization rate for aluminum is pitifully small (I work with it, I know, we remove 3/16ths aluminum that has only begun chalking after 40 years - which is the coloring coming off, the metal itself is like new). However, you'll never be able to use steel on the structure in the presence of salt water. Even insulating them won't work if the joints are capable of being covered in ocean spray as the galvanic reaction will eat the aluminum as aluminum is used as a sacrificial anode for steel.
Impressed Current could be used, but that would mean electrifying your entire superstructure, which would defeat the point of making your arcology survivable.