Here at Force 4 Chandlery we keep a good range of binoculars, great for use on and around the water. A good pair of binoculars is always handy to have on board for a whole range of reasons.
A majority of the marine binoculars we stock are 7x50 - meaning the magnification level is 7x that of your normal vision and the end lenses, or objective lenses are 50mm in diameter. This is the most common size you see for boat binoculars. Higher levels of magnification would require a very steady set of hands or stable environment, which a boat typically isn't! On top of that, higher magnification levels decrease your field of view, making seeing smaller objects at sea difficult.
There is a great choice of binoculars and monoculars available at all budgets, packed with great features such a being waterproof, floating, nitrogen filled (to prevent fogging) and some come with built in compasses allowing you to take sight bearings whilst using the binoculars on your boat.