Short river cruises and sunset cruises along the Phnom Penh riverfront are easy to arrange and offer an interesting view of the city. A cruise typically takes about 1 - 2 hours and runs up the Tonle Sap River along the central riverfront area providing a picturesque view of the Royal Palace, National Museum, parks and Phnom Penh skyline, and then across the Tonle Sap and up the Mekong River to view floating fishing villages.
Tourist boats of varying size and quality wait along the riverfront, usually between Street 144 and Street 130, and at the Passenger Port near Street 104. Just look for the cluster of boats and advertising placards. From about 4PM-5:30PM you can get a sunset cruise for $4-$5/person on a shared tour boat.