Overfishing (or over fishing) threatens ocean health by reducing fish stocks. It can upset an ecosystems, and make it difficult for a formerly abundant species to recover.
In an apparent case of "tragedy of the commons" commercial fishers generally have more incentive to deplete the stocks, than to practice moderation (in which case someone else will probably deplete it). Appropriate governance is needed to enforce restraint. At the same time, improved knowledge of fisheries can help in taking targeted action, such as protected zones, and protecting essential species and essential fertile stages in a fish's life cycle. Overfishing reduces fish stocks below an acceptable level. This can occur in any body of water.
Subsidized fishing is particularly damaging - it is a "perverse subsidy" in that it encourages undesirable behavior without offering a net benefit.
Approaches to mitigation are outlined at Wikipedia: Overfishing #Mitigation, including quotas, underfishing (to increase catch in the long term).
Growth in fish stocks can be supported through an increase in breeding areas, through protecting and creating wetlands, protecting natural reefs and creating artificial reefs (e.g. by sinking old ships). Fish thrive in structured habitats.