Out of curiosity I was looking at the Shape Water cantrip and I was wondering if this spell would be usable to create weapons. For instance if I used the cantrip first to move the water into the shape of a dagger, and then to freeze it, what properties would said dagger have?
Yes, but they will only be as good as your DM believes them to be.
Does it qualify as an improvised weapon?
Shape Water allows you to shape water into the form of a simple object, and to freeze this shaped water solid, allowing you to create ice objects.
The rules on improvised weapons have this to say about an object's eligibility to be used as a weapon:
Sometimes characters don’t have their weapons and have to Attack with whatever is at hand. An improvised weapon includes any object you can wield in one or two hands, such as broken glass, a table leg, a frying pan, a wagon wheel, or a dead Goblin.
As long as the ice you've created isn't so big that you cannot wield it in two hands, you can definitely try to use it as an improvised weapon.
What properties will it have?
The rules on improvised weapons have this to say about the weapon's properties:
Often, an improvised weapon is similar to an actual weapon and can be treated as such. For example, a table leg is akin to a club. At the GM’s option, a character proficient with a weapon can use a similar object as if it were that weapon and use his or her proficiency bonus.
An object that bears no resemblance to a weapon deals 1d4 damage (the GM assigns a damage type appropriate to the object). If a character uses a ranged weapon to make a melee Attack, or throws a melee weapon that does not have the thrown property, it also deals 1d4 damage. An improvised thrown weapon has a normal range of 20 feet and a long range of 60 feet.
This makes it clear that just how good your new weapon is depends on your DM's opinion of the object's suitability as a weapon.
You can for sure create an ice sculpture in the shape of a weapon, but the spell makes no mention of allowing you to freeze it in a way that would reinforce it or otherwise give it properties outside than that of regular ice, meaning you cannot make anything more durable than regular ice using this cantrip to freeze water, unless your DM rules that Shape Water has the ability to make super-ice in their campaign.
An ice blade would surely shatter on impact and unless your DM allows you to make it exceedingly sharp, it is not likely to cause any slashing or piercing damage.
You could try to create a heavier weapon to cause bludgeoning but a handle made of regular ice couldn't handle the stress of swinging the weapon. This being said a big enough chunk of ice, such as the head of a maul, has enough mass to inflict real damage on hit.
All of this being considered, the most likely properties of your ice weapon would be to:
- break after very few uses
- deal 1d4 bludgeoning damage
Making it sturdier
You could try to get around the lack of a proper handle and make an okay weapon by freezing the ice around another solid object such as a wooden handle, but in that scenario you would be better off using the more solid object as the weapon itself.