Oxullop , think of the low horizon afforded by the conning tower of a submarine, the mobving sea which would lift the boat up and down , the water which would be thrown up , the similar movement of a potential other ship and a picture should be forming in your mind of some of the practical problems in sighting a ship at long range from a submarione.
The standard glass was 7x50 , it afforded the best stability and detaiul for a hand held binocular.

The better the conditions the better the chance in poor conditions ranges would be shorter - you wpould always be limited by your event horizon , you could at best be looking for a shadow , a trace of smoke , a mast head on that horizon.