[RPG] What are the limitations on the Shape Water cantrip

Briefly, no.

If you're going by physics, conservation of energy means that the energy from the vacuum bomb explosion has to come from whatever moves the water, which is the spell. Because the spell doesn't have the force to damage, any vacuum bomb you make also won't have enough energy to deal damage.

D&D is a tabletop RPG, not a physics simulator

D&D is a game that aims to achieve some semblance of balance, and is directed toward being a game. Additionally, the laws of physics don't always mesh with the rules (look at the lack of rules for damage from falling objects).

There are lots of magic effects that could create huge destructive effects from air pressure alone. For example, what about Misty Step? Wouldn't you leave a huge vacuum in the spot that you teleport from, and displace a ton of air where you appear? Could you open a portable hole or bag of holding at low altitude, then open it at high altitude to make a blast of wind? If you cast Gate and make portals between two areas of different altitudes, wouldn't the pressure differential make a powerful, constant wind (see the second to last question)?

These things don't happen in the game because it's a game, and because it's literally magic. If you think too much about the physics of D&D's magic, things break down very quickly. Instead, the world of D&D uses a kind of intuitive fantasy logic.

Of course, this is all up to your DM. If your DM thinks that making a vacuum bomb is a clever, justified use of the spell, then he/she can let it happen. I personally have let a few interesting interpretations of physics slide, just because I thought it would be cool (Fabricating a chunk of radioactive metal into a shape that immediately goes critical, maybe). However, such rulings are DM fiat, and not anything explicitly supported in the written rules.

One Line of This Spell Makes These Ideas Difficult

There is an essential phrase you (originally) left out of your description of the Shape Water cantrip (Elemental Evil Player's Companion, p. 21, bold added).

• You change the water’s color or opacity. The water must be changed in the same way throughout. This change lasts for 1 hour.

All of your ideas seem to involve selectively changing part of the water to one color, and other parts to another color (e.g. changing the water on the legal document to spell out different words in some places, changing the color to be white in some parts of the die and black on others, changing the water to be white on some parts of the card and red on others to represent an ace, etc.). But since "the water must be changed in the same way throughout," this would most likely be impossible.

It might be possible to write a specific message in water on a paper or card, and then turn all that water black and opaque (or clear) as you desired. But you couldn't alter the message to fit your needs. You could argue that you can "cause the water to form into simple shapes and animate" and thus rewrite the messages or cards as you see fit. But "simple shapes" tends to mean polygons (i.e. closed shapes like triangles and squares), and usually would rule out more intricate shapes like writing.

Finally, it's quite debatable whether a wet card would be considered "an area of water" (or whether a wet document would be anything other than suspicious). You've suggested (in a comment) that the card would not be wet, but covered in "a light layer of colored frost." Note, however, that the spell does not give you the ability to melt ice, but only to freeze water. And ice cannot be moved or altered in appearance or shape by this spell: only water.