Is it treated i.e. tanalised? If it is, seal the ply with epoxy wood sealant and paint it - the treatment can cause corrosion when in contact with ali and seawater... Should be OK if it's painted though. Other option is glue some rubber around the edges where the contact points with the ali are, that would do it too.
I have painted it with an epoxy paint. I'll do another coat in a few days.
