Though Adam didn't mention his source, a little detective work revealed that the image is entitled "Balneum Sulphatara", folio 4 of Valencia Bibl. Universitaria MS 860 (formerly 138). And I had seen it before: an extremely over-exposed black-and-white version appears as plate 62 of C. M. Kauffmann's classic 1959 "The Baths of Pozzuoli: A Study of the Medieval Illuminations of Peter of Eboli's Poems". But really, you'd barely recognize them as the same.
Unusually, Valencia MS 860 has good date and provenance information for it. Kauffman (p.82) says that De Marinis dates it between 1455 and 1458: and that it stayed in the "Aragonese royal library in Naples until the Franco-Spanish conquest of 1501", when it moved to Spain until the present day. Kauffman also asserts that it was derived from the Bodmer (Geneva, Bibliotheca Bodmeriana) De Balneis MS, which he dates to the "third quarter of the fourteenth century", and placed as "Southern Italian".
If you compare the Bodmer Balneum Sulphatara drawing (f.3, Kauffman plate 21) with the Valencia one (f.4, Kauffman plate 62), you can see the reasons why the former was very probably the source of the latter: every figure is reproduced between the two MSS, each with extremely similar size and orientation. Their differences are merely ornamental: the wooden bath sides got upgraded with a fancy fish-like motif in the Valencia MS, while the top edge of the cave has taken on a stone-like 'wolkenband' appearance there.
But the big question: was either of these also a source for the Voynich Manuscript? I've gone through all the plates in Kauffman really closely, and I have to say that on that evidence I really don't think that the water section of the VMs is an enciphered De Balneis. However, I am quite sure that the VMs' author had definitely seen a copy of De Balneis and was influenced by it when constructing his pictures, in the same way that Rene Zandbergen persuasively argues that the author must have seen the manuscript now known as MS Vat. Gr. 1291 before drawing the zodiac section.
In fact, I interpret this in terms of steganography, in that I believe the style used for Vat. Gr. 1291 was appropriated as the cover cipher for the VMs' zodiac section, while the style used for the Bodmer MS and Valencia MS 860 formed the cover cipher for its water section. Whereas the particular drawing similarities between the VMs and Valencia MS 860 simply arose from having been drawn in the same general period: correlation, but not causation.
I should close by noting that Adam McLean made his own in-depth art history study of the Voynich Manuscript, posting his results on the set of pages here. One of the most compelling similarities comes from his comparison of the lozenge-shaped tiles in the picture here: but that's a discussion for another day...