Strong oxidants will convert ...

Strong oxidants will convert the ferrous to ferric ion, which then require a higher pH to form a ferric oxide precipitate, and then you do need a filtration or clarification step to remove the precipitate.  In treating acid mine water it has been found to be beneficial to recirculate a portion of the iron sludge back into the feed to promote more rapid formation and better settling of the precipitates.  This may also help in the case of sea water, but you would need to test it if you choose to go this route.  All the best and hope you are able to do this without polluting the sea surrounding the platform.