Kessil != Navy or any other military grade component regardless of branch to include low earth orbit or deep space.
Saltwater aquariums and the respected equipment while to many seem plug and play the total system needs to be built to factor in the environmental by product. Saltwater, splash, salt creep, dust, humidity, and any other number of things. My point while not helping the OP, for which I'm sorry may be out money not to mention frustrated, is that this isn't unique to Kessil. In fact the post above talking about the Radion was different in that it was in a canopy I believe with no air flow and/or vents if I recall.
OP - I know this doesn't help you and sorry about the frustration. Not sure if there is a brand that will do better than what you have but it may be cheaper in the big picture to just turn it in and have them look it over. If you do go that route see if they can tell you why salt creep happened in the unit and would could be done to help mitigate it. Could be something simple as air flow, vent, cover, etc.