Water beds are just that - beds filled with water! There are two types of water bed, soft sided water bed or hard sided water bed. The more expensive water beds are made to lessen the wave effect which was common in the early models. Water beds are usually heated and a thermostat controls the temperature.
Divans are very common and are less expensive than other types of bed. They are made from wooden covered with material and the mattress sits on top. Many divans come with built in drawers which provide plenty of storage space. Sofa beds are sofas which can convert into beds when required. They are great for spare rooms as they can be kept as a sofa and then pulled down into a bed if you have guests.