If you want to get the best and longest life from your waterbed then you should as a matter of course buy a waterbed topper. Waterbed mattress pads will simply protect the vinyl in the mattress from degradation over time. The vinyl in tends to be extremely tough but prolonged contact with human body oils can result in deterioration of the mattress but using a topper can reduce this impact by offering an additional layer of protection to the mattress.
A further consideration if you are contemplating buying a waterbed mattress pad is that purchase can reduce the need to buy other accessories. Due to the body of water within the mattress a mattress can often be cool to touch when you sleep on it, especially if the waterbed is unheated. In a warm climate this can be great, however, if there is a chill in the air it can be uncomfortable. A waterbed topper can be just the item you need to keep the chill off the bed on cooler nights offering a layer of protection between you and the vinyl mattress.
Alternatively if you live in a cool or temperate climate a heated waterbed mattress pad can be a great alternative to a waterbed heater. A heated topper thermostatically controlled or operated by a timer like a heated blanket can do away with the need to buy a waterbed heater, so not only will the topper make your bed more comfortable but can also reduce costs when it comes to buying other accessories.
If enjoy the comfort of a waterbed, then you also know that just putting your sheets on top of the mattress is not enough. Even though many of these come with a polyester/cotton cover, you still should consider getting a waterbed mattress pad or cover to protect you from the vinyl.