The softsided waterbed, after having made it's debut in the mid 1980's has surpassed it's hardside, or wood frame counterpart in sales for nearly the last two decades. Many different manufacturers have come and gone, all offering slight variations and innovations in design. Now, just a few strong players remain in production, having distilled their most popular and sucessful models down to three basic design types which we will explore here with relevance to sex; The deepfill (8" depth), The midfill (6" depth), and shallowfill (6" depth).
The deepfill is the softside type that is most akin to a hardside waterbed, as they are about the same depth and can be offered with the same watermattress types, and both require a waterbed heater. Depending which type watermattress is ordered will determine the amount of motion. Some models offer pillowtops, others come with a plushtop. The plushtop is thin, and leaves very little between yourself and the vinyl watermattress, while the pillowtop provides a soft, billowy buffer. Other than that, with regard to sex, the same rules apply as with wood frame waterbeds with one exception; the soft sides are made of foam instead of wood and can be much more forgiving in the event of any accidental impact (giving new meaning to safe sex!).
The midfill and shallowfill softside waterbeds are a hybrid of a waterbed and an innerspring bed (what we like to call a "dead bed"). Because a waterbed needs 8 inches of fluid support, these beds augment this with the use of foam. A midfill will use 6 inches of water, buffered with at least two inches of foam, and a shallowfill will have 4 inches of water, in either tubes or a watermattress, with a minimum of 4 inches of foam above and/or below the water. Heaters are optional on midfill waterbeds, and are NOT recommended on shallowfill waterbeds. All three types can be purchased with a plushtop or pillowtop, and some high-end models offer visco memory foam toppers. These beds are more comparable to regular mattresses because they move less, and allow for better "traction". Instead of using springs for base support, which will compress in the same place nightly from your bodyweight, and eventually sag, they use water, which will resilliently feel the same in 20 years as it does today, topped by a soft and supple foam or pillow top. It really feels great! They all will have some rocking motion, but the shallower the fill, the less action. Tube beds have almost no side-to-side motion, but will still have some head-to-foot motion, so you can still "go with the flow" on all types. (Of course, playing a Barry White cd may enhance your experience.)
I am now confident that, given the information here, you can make an educated and informed choice of which type of waterbed will best suit your needs for practicality, comfort, and fun. Please post any feedback or questions you may have on this or any other relevant subject.