Now, having followed some leads from Michael J. Hurst's 2007 post "The Ace of Cups: Christian Content in Tarot," I want to revisit some of these questions.
First, the PMB card turns out to be more Eucharistic than I had imagined. Consider this German woodcut of the late 15th century, called by the National Gallery, "Allegory of the Eucharist."
Note the structure of the central object: A pillar rises up from the center of a hexagonal basin; two spouts on the sides of the pillar pour out liquid; and at the top is a dove. The Eucharistic intention is obvious, since it is a fountain of blood with what are apparently Hosts floating in it. (Hurst provides a black-and-white version of this picture and suggests that the two streams of liquid represent the water and blood that flowed from the pierced side of the Crucified; in color, though, we can see that it is all blood.)
Now compare the image above with the Ace of Cups of the PMB Visconti-Sforza deck.
This is an extremely close match. The main differences are that the basin now has a stem and base like a goblet, and that the liquid is water rather than blood.
So why is it water? Why do all these cards have a design that shouts this is the blood of Christ, only to show it as water instead?
In my 202 post, I noted how similar the Marseille Ace of Cups is to this late 15th-century Spanish monstrance.
One important difference, though, is that the monstrance has no bowl -- it lacks the very part of the Ace of Cups that makes it a cup! That's why I conjectured that it must be some sort of combined chalice-and-monstrance, even though I scoured the Net in vain for images of any such object. (I did find combined chalice-monstrances, but not with a design at all similar to that seen on the Ace.) And the question still remains as to why water -- blue, definitely not wine or blood -- should be shown flowing out of the object.
Well, it turns out some old baptismal fonts look a lot like the Ace of Cups -- a large basin on a stem and base, with an ornate "Gothic" cover.
So it appears that the Ace of Cups combines the imagery of chalice, monstrance, and font. The blood/wine/water ambiguity is not entirely unbiblical, either. Several passages present the blood of Christ as something we might drink or wash in; Christ "bled" water as well as blood on the cross; and of course there is the famous miracle at Cana, where water was turned into wine. Jesus connected drinking from a cup with being baptized when he said, "Are you able to drink the cup I drink or to be baptized with the baptism I am baptized with?" (Mark 10:38). In Gethsemane he referred to his coming passion and death as a cup to drink from, and Paul later said baptism was a symbol of Christ's death -- we are "buried with him in baptism" (Col. 2:12). Mormons in particular will appreciate the connection, since the Mormon Eucharist uses water instead of wine and is said to be a renewal of one's baptismal covenants.
Waite's card also, I think, contains both baptismal and Eucharistic imagery.
As I mentioned in my earlier post, a dove flying over a chalice with a Host in its beak is very common Eucharistic iconography. On Waite's card, though, the dove is flying straight down as if it is going to dip the Host into the water. Since the Host is the body of Christ, this corresponds to the baptism of Jesus -- which was also the occasion on which the Holy Ghost descended in the form of a dove. Waite has five streams of water where we would expect four, but if water and blood are interchangeable, they could be a reference to the Five Wounds of the Crucified.
The dove flying over water also appears in the story of Noah, and it is perhaps significant in the present context that after the Flood, Noah invented wine and got drunk. In 1 Peter, baptism is connected with Noah's ark, in which "eight souls were saved by water -- the like figure whereunto even baptism doth also now save us" (3:20-21).
This biblical connection between baptism and the number eight is probably why Mormons baptize at the age of eight, and why so many baptismal fonts are octagonal. It is curious, then, that the German woodcut and the Visconti and Marseille Aces all show a hexagonal structure. Hexagonal baptismal fonts are extremely uncommon, and chalices are round, so where does this come from? Well, according to this post at the Liturgical Arts Journal, the very oldest extant monstrance, dated to 1286, is hexagonal and is an extremely close match for the Marseille Ace.
So I think we have to say that the Ace of Cups combines elements of three different sacred objects: the font, the chalice, and the monstrance.
Synchronistic postscript: After writing the above, I ran an image search on holy grail medieval drawing to see if the Holy Grail was every depicted as an object similar to the Ace of Cups. The search was useless -- most of the results were modern drawings and showed a simple chalice -- but one result got my attention because it appeared at first glance to show a chalice with a Gothic cover. What it actually turned out to be, though, was a combined image of the Grail, a castle, and my friend the owl.