50 mm lenses allow enough light to enter so that you can see through the binoculars even in less than ideal lighting conditions. This is especially useful when docking or leaving the harbor early in the morning or late at night.
The binoculars have 7x magnification, which is optimal for maritime use. This gives you high enough magnification to see what you need. Higher magnification can make it difficult to focus when you are standing unsteadily in a boat. That's why 7x is great for boaters!