HI julejlm.
Welcome to The Mattress Underground Forum! 
and thank you for your kind comments.
You were very fortunate to get 30 years of use from your waterbed. As you can probably imagine there have been many changes in the industry since you purchased it and unfortunately most of them haven’t been for the better ( see post #3 here ) so you’d need to proceed with some caution.
Liking a latex bed as a teenager is a good starting point but I’d take into account that as we age our bodies can change needs and perhaps, in your case, preferences. During the 30 years, you used your water/memory foam mattress your body got “hard-wired” into a certain feel and may have some trouble readjusting to a new sleep environment no matter how performant latex, as your material of choice is. Have you already tried a latex mattress? If not, I’d recommend you do so. Trying a few latex mattress configurations would also give you a good indication of your comfort/support needs and preferences. I’d recommend you keep good notes of your experiences as the feel of a latex mattress cannot be translated not the feel of a memory foam waterbed as there are too many variables involved
It’s important when you are testing for pressure relief or alignment to make sure you lie on a mattress for long enough that your mind and muscles are fully relaxed. A mattress can feel very different when you are fully relaxed than it does when you are tense. For most people, this means spending at least 15 minutes on a mattress that you are seriously considering and focus on the relaxed feeling that you have when you are going to sleep.
The second key is to focus specifically on testing for alignment and its symptoms rather than comfort in all your sleeping positions.
Generally, two different types of mattresses are not easy to compare (if at all) in this case Latex vs waterbed feel is different and so are the intrinsic qualities of each of the components used in the bed’s componentry. There is little value in trying to approximate the performance you got used to in your old mattress. This is certainly possible if the manufacturer has enough experience and knowledge designs and builds a mattress that is reasonably close to the softness and support of another mattress that is known to them and this is confirmed with their own personal testing (and preferably the testing of a larger group of people like their customers as well) but you have many other options available to ensure suitability that I’d be tempted to suggest that you make a “clean break” with your old mattress and start anew looking for what you really prefer and need in a mattress that would last you for 10 years or so.
Post #10 here has a step by step process that can dramatically increase your odds of finding your “ideal” mattress that has better quality and value than anything you are likely to find with a major brand or typical mass-market outlet
I agree that Latex is a very good choice of material as it has both the ability to form a pressure relieving cradle more than other more commonly used materials, be supportive at the same time as it is the most resilient of all the foam materials (although springs are more resilient than latex and some types of latex are more resilient than others). Resilience is related to the ability of a material to store and return energy)
The first suggestion I would have is to start with the tutorial post here which has all the basic information, steps, and guidelines you will need to make the best possible choices … and know how and why to avoid the worst ones which would include the major brands such as Serta or any mattress where you aren’t able to find out the quality of the materials inside it.
In the current market, it’s a good idea to avoid the major brands because they use lower quality and less durable materials in their mattresses than many of their smaller competitors although I would also avoid any mattress regardless of the name of the manufacturer where you aren’t able to find out the quality and durability of the materials inside it to make sure there aren’t any lower quality materials or “weak links” in the mattress (see the Mattress Durability guidelines here ).
You may have already read this, but choosing the “best possible” mattress for any particular person really comes down to FIRST finding a few knowledgeable and transparent retailers and/or manufacturers (either locally or online) that sell the types of mattresses that you are most interested in (latex) that are in a budget range you are comfortable with and that you have confirmed will provide you with the all the information you need about the materials and components inside the mattresses they sell so you will be able to make informed choices and meaningful comparisons between mattresses and then …
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Careful testing (hopefully using the testing guidelines in the tutorial ) to make sure that a mattress is a good match for you in terms of “comfort”, firmness, and PPP … and/or that you are comfortable with the options you have available to return, exchange, or “fine-tune” the mattress and any costs involved if you can’t test a mattress in person or aren’t confident that your mattress is a suitable choice.
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Checking to make sure that there are no lower quality materials or weak links in a mattress you are considering relative to your weight/BMI range that could compromise the durability and useful life of the mattress.
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Comparing your finalists for “value” based on #1 and #2 and all the other parts of your personal value equation that are most important to you.
If you test out any particular items locally, I’ll be interested in learning of your reaction and any questions you may have about them, or other more specific questions that you might have.
Phoenix
Note* I will be changing your post and my reply to it to a different thread as there may be more visitors interested in transitioning from waterbeds to other types of mattresses.