SihanoukVille, Kampot, Kep and Koh Kong all have day trips to Cambodia's Islands.
Hundreds of people a day end up on an island or 3 in SihanoukVille, where it costs just $15 for a morning to evening trip. Stopping at 2 or 3 islands, swimming, snorkeling, fishing, lunch, snacks, drinks, (and usually pickup at your hotel and breakfast) are all included in the price.
$25 gets you a stop at 2 islands in a 3 story party boat, with bar and restaurant and lunch included.
For $20, you can get a round trip in a speedboat to the islands and back in one day. You can book online here
Koh Kong doesn't have any regular service to the islands, but several guesthouses in town offer charter boat trips to the islands.
Kep and Kampot both have daily boat trips as well.
Also, in SihanoukVille, an occasional Booze Cruise tours around the islands, from JJ's Bar, Utopia or another beach restaurant. They include a free drink to get you started, a stop at an island for dinner (included) and you can buy other drinks on board. These cost around $10. Just don't forget your life jacket!
These all have to be booked while in town, and do not accept reservations, unless you have a big group.