The four little screws only hold the box in place. it is a self contained, enclosed box, well at least it should be, except for the leaks you will find along the edge or at one of the screw holes.
No worry about incoming water, the exhaust hose has a large above waterline loop in the stern. The box itself does have a seacock to empty it (into the bilge) but it will only have about a gallan in it. You can leave it shut (especially if it is corrodid like mine and others I've seen.) Just take off the two exhaust hoses, mop up any water with a towel.
I heard of two other C320's with the problem. One had leaks at or in the four screw holes. That fix was a little more complicated. He, used epoxy to completly close up all four screw holes, then re-drilled them. The four screws don't do anything except keep it from vibrating around.
I took it out on the dock, washed it up, let it dry in the sun, filled it full of water, then looked for leaks.
I heard of Catalina replacing one, but they wanted the original shipped to them so they could "replicate" it. ?? I guess they are not exact standard size on all C320s.
What year is yours?
BTW, I have lived on mine for many summers and one winter from The Virgin Islands, P.R., Mona Passage, D.R., Bahamas, Gulf Stream, FL, ICW. She's a great "little" boat. I'm currently in FL, Bahamas bound. We had some nasty "seas" in the Chesapeake a few weeks ago.
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