Conditions to be entitled to receive overtime pay

Dear PAO,

Prior to my resignation, I worked at a domestic shipping company involved in the delivery of consignments from Metro Manila to different parts of the Philippines. I would like to know if I can receive overtime pay since I stayed in the cargo vessel for at least three months whenever I am called for duty. Although we are not working beyond eight hours, we can only leave the shipping vessel once all the cargoes are delivered. I am now demanding my former employer of overtime pay before I sign the quitclaim they sent me. Am I legally entitled to overtime pay?