This episode is very creative and unexpected. The mandatory fishing license is a classic example of the humour that futurama uses apart from character humour such as Bender putting sunblock on Amy but only so when she' sburnt that her back reads "Honk If Bender Is Great". THe real story is when the giant fish pulls the Planet Express Ship into the water and it is not long before the Planet Express crew discover the lost city of Atlanta (now that's unexpected). The suppository pressure pills that Farnsworth keeps asking the crew to use is a great joke as is Leela going on about Fry having ocean madness. (Farnsworth: Now Leela, Fry may have ocean madness but that's no excuse for ocean rudeness). My favourite scene would have to be when Zoidberg's house burns down underwater:
Zoidberg: How could this happen?
Hermes: That's a very good question!
Bender: Oh, so that's where I left my cigar.
Hermes: That just raises further questions!