"Trying to find a good electronic horn for the trawler - thinking about an actual air horn - little compressor size of a beer can - are they more reliable than straight electric?"
Check out http://www.hornblasters.com/
I installed one of these, and also a Kahlenberg horn that I got off of eBay. A few notes:
1) Depending on your boat, you may just want to install a standard 110 VAC air compressor in the engine room and use that to power the horn. I put in an inexpensive Porter Cable pancake-style compressor, and use it for a all kinds of things. Runs just fine off the inverter when underway.
2) The first horn I installed (one of the train horns) uses a solenoid. It's easy to install, but has the disadvantage that the only volume is LOUD. For the Kahlenberg, I installed a train air valve, like this: http://www.hornblasters.com/products/details.php?i=half-inch-manual-train-horn-valve This is much more practical because you have a lot of control over the volume.
3) I discovered after the fact that boat horns are actually tuned, and according to the Colregs you are supposed to use a particular frequency based on the length of your vessel. Train horns, while fun and loud, do not necessarily meet the specifications. Here's a link: http://www.kahlenberg.com/imo.html My Kahlenberg, I discovered, is only supposed to be installed on a vessel over 75 metres (about 150 ft). Oh well!