As the name suggests, subsea is a common term that offshore workers use to describe something that is underwater. The distinction is there to class something as specifically being under the surface of the ocean. This is an important distinction, as it implicitly defines a number of conditions, them being -
The item in question will be:
Exposed to salt water.
Subject to currents and tides.
Located beneath the surface of an ocean (or sea).
Designed to be located underwater.
The final distinction means that a particular part of an offshore structure might have been designed to exist above sea-level, but is underwater due to a particularly high tide or heavy seas. As such it isn't a subsea component, it's a submerged, topside component.
The only general exception to this, is something that was not designed to go under the surface, but nonetheless now resides there on a permanent basis. This includes parts of the structure affected by rising water levels, or ROVs that have been lost in the shadowy depths.
A subsea development in the oil and gas industry describes a field whose wellheads and field infrastructure are based primarily on the seabed. This is as opossed to jackets and drill rigs which do most of the work topside.
Also referred to as sub sea and sub-sea.