The following website,http://www.geimin.co.uk/assassins-creed-rogue-ship-locations...
has a a little section on where the POW spawn (although no indication on any trigger points to get them to spawn).
It list the locations as follows,
1- East of Glace Bay
2- East of Chameau
3- Northwest of Pearl Island
4- East of Port Meinier
5- Northeast of Harbour Deep
6- South of Fogo
I have encountered POWs a couple of times at number 3, so I believe the list to be true.
Hope this helps refine your search for these dam ships