The Goa Tourism Development Corporation (GTDC) runs different river cruises on the Mandovi river during daytime, sunset, and moonlight. If you want to take a day cruise, there are two kinds- one from the Panaji jetty down the Mandovi into the Zuari bay and the second is up the Mandovi to Aldona and a mineral water spring.
There are also a few private firms that offer a variety of exclusive cruises or boat tours for sunset views as well as for crocodile or dolphin spotting.