Rashi: Do good and Chesed to someone that you think that you will never see him again. This is like sending bread on water (you will not gain anything from it).

Ibn Ezra #1: This exhorts one who has [wealth] to be generous. His hands should be open to people that he knows, and that he does not know.

Ibn Ezra #2: Some say that the water is a pond with fish ¹ .

Ri Kara: Sow your seeds in a place of a source (spring) of water. Even if others' grain will be stricken the entire year due to drought, you will not fear this.

Rid: Whatever you can benefit others - do so. Do not say 'for whom do I squander my money?!'

Seforno: Even though the king and officers do not benefit others like is proper for them, you should not refrain. [Boats] go quickly on water to distant lands.

Orchos Tzadikim (Sha'ar ha'Nedivos DH v'Da): The simple meaning refers to generosity. One who sows Tzedakah, he reaps its yield.

Metzudas David: Do not think that the water will take it far away, and you will not find it. This is a Mashal. Give food even to one whom you do not expect to see him again; in the end, you will get reward from him.

Rashi: [If you benefit one whom you do not expect to see him again,] you will receive reward ¹ ! Yisro said "Kiren Lo v'Yochal Lachem" (Shemos 2:20); he thought that Moshe is an Egyptian, and he will not see him again ² . In the end, Moshe became his son-in-law and the king of Yisrael; he brought Yisro under the wings of the Shechinah, and his descendants merited to sit [on the Sanhedrin] in Lishkas ha'Gazis.

Ri Kara: At the time of the harvest, you will find that what you seeded, it produced fruits.

Seforno: If you persist [in benefiting everyone], you will acquire a reputation; it will give to you grace in the eyes of Elokim and people.

Orchos Tzadikim (Sha'ar ha'Nedivos DH v'Da): Tzedakah to Aniyim with Yir'as Shamayim, is a treasure that is never lost. It stands for all days.