Marlin, sailfish and roosterfish are on a catch and release basis only.
You may keep all meat fish you catch: tuna, dorado (mahi-mahi), wahoo, snapper and grouper. The mate will professionally fillet and bag it for you on the way in. Most restaurants in the area will be happy to cook it up for you.
It is customary to offer a bag to each the captain and mate if there is extra. In Costa Rica it is illegal for the crew to sell fish, the boat can lose the license if caught. However, if there is unwanted excess, the crew usually know people to share it with.