Well, the stock horn FW installs is the Fiamm 519 series. I understand Fiamm is right across the street from FW in Cadillac, MI. Their website doesn't have much info on these horns, but it does have pics of the covers and a wav file so you can hear what they sound like (of couse we already know what they sound like). Contrary to my belief, the description below indicates that the horns do meet C.G. specs. Sure don't seem like you could hear them a 1/2 mile away, but I guess sound carries over water.
http://www.fiammamerica.com/Product.asp?ProductID=281Most of the aftermarket drop in style horns have oval covers, but the Fiamm is round. {
Correction the Fiamm's are oval too 
} Wonder if the hole size is standard? I suppose adding a second horn is an option if the hole size is non-standard, the Fiamm horn might not seem as wimpy x2. FW says they should cost about $25 from a dealer.
Here's the specs from their catalog:
519 Series Hidden Horn
Easy to install
Pre-installed gasket
5-year UV rating
Limited Lifetime Warranty
Our Hidden Horns blend neatly with today‘s cleaner, sleeker boat designs. Only the attractive grills in black, white or stainless steel are visible. Made with top-of-the-line materials and workmanship, these USA manufactured horns will provide years of trouble free operation and the best sound quality available in the market. All FIAMM Marine horns meet or exceed ABYC, NMMA and Coast Guard requirements and are NMMA type accepted.
Part No.(Boxed)Description
5190011 Horn w/ SB White Grill
5190111 Horn w/ Panama White Grill
5190211 Horn w/ White-White Grill
5190311 Horn w/ Fishwhite Grill
5190411 Horn w/ Black Grill
5190511 Horn w/ Stainless Steel Grill
WORLD’S LARGEST HORN FACTORY 1550 Leeson AvenueCadillac, Michigan 49601 Telephone/231.775.2900 Fax/231.775.4402
http://www.fiammamerica.com Cadillac, Michigan