A vessel on the sea often refers to a ship or boat on the ocean. By definition a vessel is a craft designed for water transportation
Under Part A of the International Regulations for Preventing Collisions at Sea (COLREGS), the term “vessel not under command” means a vessel which through some exceptional circumstance is unable to manoeuvre as required by these Rules and is therefore unable to keep out of the way of another vessel.
A vessel is a ship or boat, or a tube in a body that carries fluids.
CLOT
In a storm a sea anchor can help a vessel from drifting off course. It will keep the bow in the wind and slow the speed of the vessel.
The term "sailing" is also used for submarines - the definition of "sailing" has several meanings besides the one most people think about, which is wind billowing through canvas of a sailing vessel. The one that applies is "The skill required to operate and navigate a vessel". It doesn't make any difference if the vessel is above or below the waterline.
A vessel at sea is said to be underway.
A vessel is defined as "any craft larger than a row boat". So a yacht fits well within the definition of a vessel.
A Ship
what are sea-going vessels
BATHYSPHERE
a ship
A submarine is a sea-going vessel, both on the surface and under the water.