Noted a discussion with our own RichPowers about this on another site.
I had a Decade MS-100 inside a weatherproof box on Bowen Island pretty close to the water, running 24/7. Absolutely no damage at all to the transmitter - I did note that the metal screws securing it to the box were rusted after 2 years.
I also put heat shrink tubing over the telescopic antenna that was sticking outside the box - it was like new when I removed it.