ok there is one that respawns really fast and always as a maurder near outcast just sail around there till u find it. sink it about 30 seconds a new one will appear and so forth
for the barracuda you can find one that respawns right off the coast of peridia and sails to cuba and back to peridia WARNING: it is a flag ship and will always respawn as a flag ship! occasionally u will get about 4 mercs but mostly u get hire-guns so be ready!
for corvettes one respans to the left of tortuga if u are sailing away from tortuga this respawns ever 15 seconds afetr death
for sea vipers they respawn all around the devils anvil
hope this helped all people who need to find these ships
backing up what ETM said there are literally 30+ threads about this try using the search button
just a heads up