I just picked up a boat and the trailer axles have zerk fittings, can I or should I use bearing buddies with these?

no, the bearing buddy pushes grease in from the outside of the axle, if the zerk is in the end of the axle, you just grease it and the grease goes through and comes in through the back and forces the old grease out. take the metal caps off or the rubber plugs and inspect, feel free to put more in and make sure they are lubed. i just replaced 4 hubs and redid all 6.

Make sure the rubber caps are not dry rotted! They are cheap @$2-3 each.