Yes the men on watch duty did use binoculars while on duty ,standard was 7x50 made by several manufacturers such as zeiss, jena etc. The stanard watch was four hour shifts with a watch officer and four lookouts, this could change with the weather or battle condititions. It was usual for the next watch to releave the standing watch 15 minutes early as a custom. In general on a good day a u-boat lookout could see for about 6 miles,less for fog,rain ,heavy sea or snow storms etc. By interogation of German U-boat prisoners the British found out that the U-boat watch on average cleaned their binoculars for a total of a half an hour in a four hour shift. Binoculars were cleaned with a leather chamois cloth either on the bridge or were sent below,when this was done a fresh pair was sent up to him. Since there were four men each searched a 90 degree area and under no time was he allowed to take his eyes off of his sector, even if they were sinking a ship he could not watch as it went down. If the captain felt he was in a danderous area he could increase the watch and if looking for a convoy he may have the man with the best vision up on watch.