There is no real reason why you cannot use a memory foam topper on top of your water bed. However, keep in mind that most people place a foam pad on top of a firm mattress to make it softer and not the reverse. For what it’s worth, when I placed a firm travel mattress on top of an air mattress (don’t ask why), it always felt a bit “off” to me. I certainly would not recommend it. On the other hand, more than a few people have reported success in “fixing” a sagging mattress by placing a memory foam mattress pad over it. I know that this is not quite the same kind of thing, but …
Alright, so let’s say you still want to try this out for yourself. May I suggest that you buy a cheap egg crate foam topper for your experiment? There used to be a time when real single/twin memory foam toppers (1.75 inches and 2.5 inches) sold for under $30 and I recommended them without reservation. Unfortunately, most seem to sell for over $50 now, so I no longer tell my friends to buy just to try. For experimentation nowadays, I advise using the much cheaper egg crate toppers.
If you really want to try to make your water bed firmer, one alternative to a memory foam mattress topper is a rebonded foam topper. Rebonded foam is denser than memory foam and is very tough, yet cheaper. It is the main material used to make motorcycle seats, proof of its hardiness.