Why not move the heat-exchanger (thin walls and all) into the ocean.
Have a closed-loop of FRESH water that you pump around.
It cools down in the ocean and then is pumped back in, through the data centre, heats up.
Now you've got internals (and your pump) free of salt.
Yes, you still have to maintain the outside of all the pipes AND the ocean-side of the heat exchanger but that seems easier than moving salt around.