there are plenty of fish in the sea

To say there are plenty of fish in the sea means there are a lot of possible people to be in relationships with, often referring to a boyfriend or girlfriend or potential spouse. This phrase is most commonly used in reference to someone just being dumped by their boyfriend or girlfriend or being bummed out about not getting into a relationship with someone they were interested in; it is a common expression to let the person know there are plenty of other people out there to be interested in.