Waterbed mattress is a piece of vinyl filled with water or gel. Sometimes construction is more complicated (wave-reducing), but the idea is the same. There are two types of waterbed mattressess - hard-sided and soft-sided.
Pros of waterbed mattress:
Water can be heated, providing excellent comfort with temperature control.
Excellent sleeping comfort.
Very easy to clean the surfuce, so dust is not a problem at all.
Heating the water requires energy. So it constatly eats costs. Average water bed eats ~500 kWh / year.
Much harder to move / transport because of hight weight and sensitive content.
Very high weight, consider this if you have old wood floor.
Bad water mattresses are easy to broke and very dangerous if water leaking too fast, can cause a real domestic disaster.
Hard-sided waterbed mattresses sometimes are of non-standard sizes.
How to choose:
Get one with wave control system. Without this disturbed water mass will start significant water action and it requires time to stabilize. The waving can stimulate qualms if you are not a good sailor. There are several technics used for stabilizing: fiber batting, interconnected water chambers, mixed air and water chambers, watercoil technology.
Vinyl repair kit is a must.
Choose the right mattress size, because there are alot of waterbeds with non standard sizes on the market.
Try one without wave control system.
Waterbed mattress is very unusual choice. It has alot of disadvantages and advantages at the same time. If you don't have strong feeling that you need it, then just choose more common mattress type. Read on, and you will find some.