4 or 5 bed room houseboat is given as sharing basis, each couple will get separate rooms with attached bathroom in one houseboat. They can share common lobby, dining area, top floor, sun deck etc, this is called a sharing houseboat.
Just think that you are staying 3 star hotel. You are getting private room and you go for common restaurant to dining. Similar way you are staying houseboat which is floating/cruising and you can hangout around the houseboat just like hotel. There is enough space for all guests. Upperdeck area is commonly using for dining purpose.