"Porpoises" sounds like "purposes". "Educational purposes" is a common saying, meaning that something is for learning. A porpoise is an aquatic marine mammal. Marine biology is the study of living creatures in the sea.  The joke is playing on the term "educational purposes" but using "porpoises" to replace "purposes" because marine biology seminars is what is getting discussed. 
The marine biology seminars weren ' t for entertainment , but were created for educational porpoises .