Rust forming and being washed off by water
Rust appears because iron is oxidized in presence of oxygen (it is a oxydo-reduction reaction, meaning an electron transfer occurs between species), and water helps this phenomenon occur. Iron loses 3 electrons to the favour of the oxygen.

In presence of water, oxygen can oxidize iron by the mean of hydroxyl group. It leads to:

$$\ce{2Fe(OH)_3 \leftrightharpoons Fe_2O_3\cdot 3H_2O}$$

where water is not the oxidizer, rather it is a mediator for oxidation.

Hydroxides have low solubility, so water can only dissolve a small fraction of it.

On the other hand, poured water can abrade rust (via mechanical stress) and remove it of from the bulk metal, leaving the surface clean for producing more rust. This is how huge rods of metal can vanish in a few months in corrosive environmental conditions (such as wet air or water).

Preventing rust requires the use of specific alloys and surface finishing. You can also add a galvanic electrode (generally made of $\ce{Zn}$, a so-called sacrificial anode) to protect submerged metallic structures such as the hull of a boat.

Therefore, in your example water is both a mediator of oxidation and mechanical abrasion agent.