To/from Laos - There is one border crossing for tourists on the Mekong, a 90 minute speedboat ride north of Stung Treng. The border guards have few opportunities for "alternative" income, and will usually try to make a few extra dollars from scamming tourists.
To/from Thailand - There are no ferry services between Cambodia and Thailand. However it is possible to cross the border from Thailand to Koh Kong on foot, take a 15 minute share taxi or motodop (motorbike taxi) ride, and then proceed to Sihanoukville by ferry from there (or vice-versa).
To/from Vietnam - It's possible to travel between Ho Chi Minh City and Phnom Penh by boat, or by combination of road and boat. Fast boats leave daily from Chau Doc in Vietnam's Mekong Delta and take 5 hours to reach Phnom Penh. Chau Doc is a four hour drive from Ho Chi Minh City. A popular overland route is to make a three day trip, stopping at Can Tho and Chau Doc before taking the boat to Phnom Penh.