Find someone who knows about and is an expert in loaded bolts and nuts in a salt water environment and about flanges. I question using a flat face flange.
TaperLok Knows flanges but not enough about loaded bolts in salt water.
This is not an easy cook book solution.
Several companies make split sleeve repair clamps that will do the job. The first that comes to mind is PLIDCO (Pipeline Development Co., I think). They have a web site.
Put split sleeve repair clamps in your browser--you'll get more than several examples.
