Hi cope,
Welcome to the Mattress Forum! 
Unfortunately, there is no one answer of what best approximates a waterbed, as this is subjective. Additionally, you’re not looking to have an exact feel like a waterbed, but instead something different that is more supportive, which will by definition be a different product. But I get the gist of your query.
The first thing I would have you read would be this article I wrote about waterbeds and how they work, which will be useful for context of my comments. It’s a quick read.
Getting to the basics, the two main functions of a mattress are pressure relief and spinal alignment. The waterbed does a good job at pressure relief, but in many instances the spinal alignment (as in your case) can be insufficient.
I find it can be much more accurate to test and “evaluate” a mattress in terms of a comfort layer (pressure relief) and a support core (spinal alignment and support). This way the words “soft” and “firm” can take on more specific meanings depending on whether you are talking about the pressure relieving qualities (top layers), the supportive qualities (lower layers), or the overall feel of a mattress (much more subjective). This can lead to more accurate feedback in terms of where a mattress may be too firm or soft rather than a more subjective description of the mattress as a whole.
It may be worth considering something using latex in the comfort layers because it’s much more resilient (springy or bouncy) than memory foam, and very point elastic. There is more about some of the differences between latex and memory foam in post #2 here, but like anything the most reliable way to know which type of materials or mattresses you tend to prefer in general will be based on your own careful testing and personal experience. Some of the newer ultra high density high performance polyfoams are also quite point elastic and they provide a gently contouring feel that many people enjoy coming from a waterbed mattress. There is more information about these foams and their characteristics in post #2 here.
Then you need to address the type of support core to be used, as this was your main concern, as to improve support and spinal alignment. A product using a pocketed coil spring unit (what some refer to as a “hybrid” mattress) certainly can be a good choice, as it can provide good support while allowing a bit more “independence” and travel in the deeper support layer. A product using a latex support core can also provide excellent support characteristics while being very conforming (although most conformation is felt as a result of the uppermost layers of your mattress). Even something using a polyfoam core can work well. All could be good options.
I think that since you are considering moving from a waterbed to a more “conventional” style of mattress, some local testing maybe beneficial. If you’d like, you can provide your zip code and I can see if I am aware of any better options in your area where you may go test some alternatives to your current waterbed.
Also, just in case you haven’t read it yet … be sure to read through the mattress shopping tutorial here which has all the basic information, steps, and guidelines that can help you find a mattress that is a suitable match for your body type, sleeping positions, and preferences … and know how and why to avoid the worst ones.
For now, that’s the best advice I can offer, as nothing will be more reliable than your own careful testing (hopefully using the testing guidelines in step 4 of the tutorial) or your own personal sleeping experience (see mattress firmness/comfort levels in post #2 here).
Phoenix