I agree with Richard.. doesn't look very bad at all to me. I don't see any deformation that would have happened if the angle iron would be thinned significantly. In dealing with structures and pressure vessels, we used a rule of thumb that there is a 4 to 1 ratio of rust to iron loss.. so rust looks really bad mostly but it doesn't consume a lot of iron.. The rust and a little pitting in that picture doesn't look bad enough to be concerned about.. If you really want an answer that you can hang your hat on, you can drill a very small hole thru the plate, say 3/32" diameter (no bigger, and as far away from the tie rod as you can get) .. and then take a small piece of wire with a small 90 degree bend on the tip .... poke this through the hole then hook the bend on the underside of the plate ...and while gently holding up pressure on the wire, mark the straight part of the wire where it exits the top surface.. take the wire out and you can see how thick the metal is by measuring the distance from the bend to the mark. From what I see in the picture, I would do two things.. stop any water leakage (yes, it could be coming through a portlight frame, or the deck, or..) Paint the exposed part with a rust fixer or cold galvanizing..
don't clean that area with any kind of chlorine or bleach based cleaners ..