Letters of Marque are given to the player by Governors which makes them a Privateer and permission to sink enemy ships. This is the most basic level of pirating. If you say, raid a few Spanish ships as soon as you spawn, and after a while your bounty goes away. If you talk to a Spanish governor, it is likely that they will offer you a Letter of Marque for a cost. Since you spawn with four different nations, their maps are varied depending on which city / town you are in.