Q: My friend was holding money in his hand. I knocked the money out of his hand and it rolled into the gutter and down a drain. We see it lying three feet below. Must I reimburse him?

A: The Gemara (B.K. 98a) writes that if someone knocked money into clear water and it can be retrieved by a diver, he is exempt. The money is still present and the cost of hiring a diver is incidental.

Nonetheless, the Shulchan Aruch, following the Rambam, rules that you are liable as a form of garmi. However, the Rema rules that this is considered grama and you are not legally liable, in accordance with the simple reading of the Gemara (C.M. 386:1, 3). The Shach (386:7) concurs with this ruling.

If it is not possible to retrieve the coin, though — e.g. the water is murky — you are liable even according to the Rema (see Pischei Choshen, Nezikin 3:[44]).

Furthermore, if you grabbed the coin and threw it away, you are then considered a thief. You are responsible to return the coin even if it entails expenses to do so, or to pay for it (see Shach 386:8).