I think there's a tendency for such things to get ripped apart when a storm geometrically increases the ocean's might.
My thought is to find a non navigable area that has a steady current. Submerge the entire unit so that it is located below the range of wave action. (Mark with buoys, of course) You could run it simply as a tube with a turbine at one end to use the current--the simplest way.
Or if you want the ram jet, put a plane surface on each end of the tubes. The current creates lift over the surfact to tilt the tube up on one end to bob up, make the plane tilt on the other, and have a water driven cam that reverses the direction of the planes at the limit of tilt. (Like the inner workings of a water sprinkler.)