Prospex for Seiko basically means "Professional Specification" that their timepieces in this category are built with very high standards to be used by professionals. 

The Professional Association of Diving Instructors (PADI) is the world’s leading scuba diver training organization. Both started out serving the diving industry in 1965, and to celebrate this special relationship, Seiko introduced the Prospex PADI Turtle 200M Automatic Blue/Red in 2018 to the Japanese market. It is no wonder the new Prospex Turtles are so popular among Seiko and dive watch collectors.