Several guesthouses and some "restaurants" offer food, but don`t expect anything special. There's an Indian restaurant with decent curries along the main street, called "Hassan". The friendly manager will also package meals for consumption on the slow-boat. Be careful while paying though, he might tend to give you back too litle or hand you unusable too old, broken dollar-notes. NEVER accept a dollar-note that's a little ripped, it's unusable!
Locals set up small sandwich stands along the road to the waterfront each morning. You can buy basic sandwiches of bread and Laughing Cow cheese. If not, ask your guesthouse to provide you with a packed lunch, as you are going to be in for a 9 hour boat ride without meals. Some boats do have some food and drinks for sale too.