My first shark sighting happened at the same time I met my first dolphin:
https://66.media.tumblr.com/d7d16359f05585df3b7ca88ccd9dfdb9/tumblr_ptgv753eNn1t1fm14o1_540.jpg
Afterwards I've had four occurences of sharks when my sims were occupied with the dolphins, in which case both shark and dolphin just dive away. Love the animation they are doing, the distressed expression and the arm waving!
The only time a shark swam by without a dolphin nearby was when one of my favorite families was swimming in deep water, but it didn't attack. Poor thing must have known I wouldn't let it eat a member of that particular household (ambrosia or reload).