To unlock this trophy, you will be required to take out 8 of the ship captains in the game. They're located on the water and are marked on the map with the camp (spear) symbol or with a question mark. Due to these locations being on a boat, they're not at a fixed location but they stay within the same region and can be seen on the map if you've synchronized the viewpoints within their vicinity.
Lake Mareotis - North Mareotis Trireme - Level 6-8 suggested
Lake Mareotis - South Mareotis Trireme - Level 6-8 suggested
Kanopos Nome/Great Green Sea - East Mediterranean Trireme - Level 11-13 suggested
Ineb-Hedjet Nome - Upper Nile Trireme - Level 19-22 suggested
Faiyum - East Moeris Trireme - Level 24-29 suggested
Im-Khent Nome - Nile Delta Ship - Level 30-33 suggested
Atef-Pehu Nome - South Nile Trireme - Level 30-33 suggested
Kyrenaika/Mesogeios Sea - West Mediterranean Trireme - Level 34-38 suggested