The only way to find a set of air-horns that WONT RUST is buy marine grade horns. They are made to resist salt water. Even plastic horns most likely have steel diaphragms. Facing back is NO guaranty against water ingress.
STEBEL do make a Marine version of the Nautilus.
http://www.stebel.it/stebel2012/products.asp?IDserie=18&uso=1&IDprodotto=36&tipologia=nautica
" NAUTILUS COMPACT TUNING MARINE
Compact twin-tone electropneumatic horn engineered specifically for sea vessels. Perfect for small/medium sail and motor boats. Stylish Italian design allows for external viewable mountings (ex: on rollbars, etc…). Finished with “nautical white” high-grade anti-yellowing ABS.
More water proof than Nautilus Compact, it features an intricate maze system for the protection of the “anti-water spray” air intake. High decibel power and pleasant pitch that only an air horn can offer (115 dB(A) at 2 meters – 139 dB(A) at 4 inches, 530 + 680 Hz). Simple and fast assembly, does not require external air tube connections. Requires only a single fixing point thanks to its all-inclusive design. Because of its style, this important instrument compliments well the other accessories normally present on sea vessels (antennas, radar, etc..). For other characteristics see “Nautilus Compact”.
Homologated CEE ECE and Electromagnetic Compatibility "