Hi Ann-Marie Thompson.
Welcome to our Mattress Forum! 
Without personal testing (of specific mattresses and materials and finding out their specifications) and going through the learning curve of becoming familiar enough with different types of materials and layering and how they work in combination with each other to predict how a certain layering may work for you … you would be much better off simply going with the suggestions of a knowledgeable retailer or manufacturer who is familiar with how the different options they offer fits the “averages” of people who may be similar to you.
Unfortunately there is no shortcut to the process of learning about your needs and preferences especially as you are now considering a very different type of system than what you are used to.
Overall, there is no formula that can predict with any certainty what type of material or layering you may do best with that can possibly be more accurate than your own personal experience.
In your testing and research I would make sure to check the specs of each of the components in a system you are considering. This is the part of your research that you can’t see or “feel” and assessing the durability and useful life of a mattress depends on knowing the specifics of its construction and the type and quality of the materials inside it regardless of the name of the manufacturer on the label or how a mattress feels in a showroom/hotel or when it is relatively new so I would always make sure that you find out the information listed here so you can compare the quality of the materials and components to the durability guidelines here to make sure there are no lower quality materials or weak links in a mattress that would be a cause for concern relative to the durability and useful life of a mattress before making any purchase.
Whereas I understand that water beds may do well for some people and may have certain advantages such as providing good pressure relief … overtime it can also lead to back issues and pain for some people if the waterbed they are sleeping on doesn’t keep their spine in good alignment in all their sleeping positions over the course of the night. You can read some more information about waterbeds in this article here
I’m not sure what you’ve read since you found the site but just in case you haven’t read it yet … the first place to start your research is the mattress shopping tutorial here which includes all the basic information, steps, and guidelines that can help you make the best possible choice … and perhaps more importantly know how and why to avoid the worst ones.
Two of the most important links in the tutorial that I would especially make sure you’ve read are post #2 here which has more about the different ways to choose a suitable mattress (either locally or online) that is the best “match” for you in terms of “comfort”, firmness, and PPP (Posture and alignment, Pressure relief, and your own Personal preferences) that can help you assess and minimize the risks of making a choice that doesn’t turn out as well as you hoped for and post #13 here which has more about the most important parts of the “value” of a mattress purchase which can help you make more meaningful quality/value comparisons between mattresses in terms of suitability (how well you will sleep), durability (how long you will sleep well), and the overall value of a mattress compared to your other finalists based on all the parts of your personal value equation that are most important to you (including the price of course and the options you have available after a purchase if your choice doesn’t turn out as well as you hoped for).
I hope this gives you a good start and helps eliminate some of the confusion you are experiencing.
Phoenix