What is the first thing you do when carrying a firearm on a boat?

To be honest i have never needed a firearm on a boat, or spent much time on one. I do however have extensive knowledge of firearms and can offer some suggestions. First would be is this boat going to be in freshwater or saltwater?

Either way you should probably keep in in a protected compartment to shield it as much as possible from the elements, with a coat of CLP. Also for safety sake i would recommend keeping the weapon in an "Amber" status. For a magazine fed weapon that means magazine in weapon, no round in the chamber, safety on. For a shotgun, shells in the tube, none in chamber. Unless specifically needed i would avoid keeping a round in the chamber. Finally if you plan on keeping it on the boat all of the time a hidden or locked compartment might be the way to go to prevent theft.

The message here really is safety first. Also you might want to check your local laws regarding keeping a firearm in a vehicle, specifically a boat. your local Sheriff's dept or PD could also likely point you in the right direction (legally speaking of course).