Let's look at the context of the statement:
1 Cast your bread on the surface of the waters, for you will find it after many days.
2 Divide your portion to seven, or even to eight, for you do not know what misfortune may occur on the earth.
3 If the clouds are full, they pour out rain upon the earth; and whether a tree falls toward the south or toward the north, wherever the tree falls, there it lies.
4 He who watches the wind will not sow and he who looks at the clouds will not reap.
5 Just as you do not know the path of the wind and how bones are formed in the womb of the pregnant woman, so you do not know the activity of God who makes all things.
6 Sow your seed in the morning, and do not be idle in the evening, for you do not know whether morning or evening sowing will succeed, or whether both of them alike will be good.
I believe that this entire context is a discussion of benevolence. The encouragement here is to be a benevolent person, because one never knows when he might be in need of benevolence himself.
The expression "cast your bread on the surface of the waters," is taken from the custom of sowing seed by casting it from boats into overflowing rivers, or in marshy ground. When the waters recede, the grain will fall to the soil and spring up.
"Waters" here could be an expression used to represent people, many people, who are recipients of our benevolent efforts, who in turn return to us benevolence in our time of need.