The word aarti refers to a hindu worship ritual. These rituals include the use of fire, usually in the form of lanterns or candles. fire in hinduism is associated with the sacred, seen as the medium through which humans can communicate with the gods.
In india, the ganga (or Ganges) is considered not only a holy river, but also a goddess in its own right. Worship of the ganga River as a deity has its roots in hindu tradition, which believes that washing oneself in its waters will cleanse the body and mind from sin.
You may also wish to buy a diya, one of the flower-filled, candle-illuminated boats that float down the river. Diyas, as well as blessings, can be obtained from one of the priests working during the ceremony in exchange for a small donation.