What is the meaning of "difficult to cross the sea"

The sea of misery is always immersed in the unchanging worldly way, and humanity can hardly surpass the heaven, not to mention being a mediocre person who can't even realize humanity. The true spirit of the soul is that people in the boat of suffering, which is made of flesh and body, are drifting endlessly, just because it is difficult to get rid of the bondage of this world balance outside the impenetrable world.