I have seen a sherwin williams product - KEM 400 - used on a tin boat and a Jeep up here. Looked great and lasted.
Search KEM 400 on the net and you will find it referenced in several duck boat painting threads and even equipment painting threads.
Here is one comment from MudmotorTalk.com
Here is the deal on KEM-400 by Sherwin. You need to get it from a Chemical Coatings store - NOT your neighborhood Sherwin-Williams store. CC stores are found in major metro areas. When the store blends it up for you make sure they knock the sheen down to 2-5% reflectance. They can do this with additives at the time of mixing. If you do not get this done, the paint will not be flat, as KEM-400 straight out of the can is glossy. I have a gallon of the "Gator Boat Brown", the custom color that Gator Trax uses for their boats. Although the name would imply brown, this is actually an o.d. color that will not end up looking "kermit the frog" green like some o.d. green paints on the market.
KEM-400 sprays easily through an HVLP gun, dries quickly and wears like iron. Like any topcoat, it performs best over a properly prepped and primed boat.
There are a lot of good boat paints out there, but for me you can't beat KEM-400.