It's why we love them, but they don't make ideal platforms for viewing magnified images. The more an image is magnified, the harder it becomes to keep the object in view. We feel that seven power is about the maximum usable magnification for use on boats 50' and under. Consider 6x30 glasses for boats under 25', especially if you don't use the boat after dark. They are lighter, more compact, and the reduced power makes them steadier to look through.
Some users feel heavier binoculars provide a more stable image.
However, they can be tiring to use for long periods. Polycarbonate housings, like those on the Fujinon 7x30 SHS, make for a lighter weight, yet rugged, binocular. In waterproof marine binoculars, the common method is to allow each eyepiece to be focused independently, both for the differences between eyes, and for different distances. In center focusing binoculars, you first adjust one eyepiece to accommodate the difference between your eyes. A central focus knob then adjusts both sides simultaneously to focus for a particular distance.