Nick sees opal, a stranger, drowning in a pool, but does not attempt to rescue her. opal could successfully sue nick for

Opal could successfully sue Nick for Guilty of Negligence.

In the very first place, when you see a person drowning, it's in your mind to save or let them be. The person drowning may die if not saved immediately and it will be in your conscience if you were not able to save them. You know that the situation is dire but you choose nothing to do about it then maybe you'll get sued for negligence.