You acquire guns by taking them from fallen foes or swapping them out at conveniently placed gun boxes. (tomsguide.com)
Cards dropped by fallen foes are also used as part of you upgrading your characters abilities. (thegamersparadise.com)
Anyway, to help further link your naval ventures with your on land adventures you can collect Wild Cards from the bodies of fallen foes which can them be used to bolster up ship stats, such as more armor, firepower or more radar range. (wolfsgamingblog.com)