The armed group arrived in a vehicle late yesterday, stormed the house and opened fire with high-caliber weapons, officials said.
"Four men died at the scene and four more, also males, died in a hospital during treatment," Joaquin Carrillo Ruiz, chief prosecutor of Oaxaca state, told AFP.
The victims were all fishermen who caught sharks, but the prosecutor's office said in a statement that investigators were looking at whether they were involved in other activities at sea.
Authorities had earlier confirmed seven fatalities but the eighth victim succumbed to his wounds on Tuesday.
Puerto Escondido is a popular tourist destination and is beloved by surfers worldwide.