PROS:
Excellent people to meet
You now have an extra 80% of the world you can explore
You can see things few other humans have ever seen
Experience weightlessness without having to go into space
Always have something new to learn
When diving is good, it is very very good
CONS:
Some right *-ers to meet as well
you now have an extra 80% of the world to explore, so long as you enjoy swimming around in the pitch black
you can see things no other human has seen, unless you are in a part of the ocean which is cold and dark in which case you can vaguely feel things no other human has vaguely felt before
Experience weightlessness without going into space, though for most its more of a bouncing motion
Always have something new to learn, though you may learn to late.
When diving is bad, it is very very bad