So my cousin really really wants to be a waterbender and he's trying to find ways of using shape water to attack, but of course as the spell states when you move the water it isn't with enough force to actually cause damage, but then we came up with this idea, there's another person in our party who also has the shape water cantrip, can the first person hold their action so that "when player throws the water i freeze it as it's being thrown" so that on the second person's turn when they throw water at someone, it's essentially turned into a 5 ft square brick of ice being hurled at an enemy? i assume that the fact that it deals no damage as a liquid state but would that be different if it's changed to a solid state as it's being hurled at an opponent?

also, could you use shape water to drown someone? ie move the water over someone's head so they can't breathe? for extra measure because i'm sure many would say that the creature could just move out of the space that the water is in, what if they are being restricted by something, for example when they're under the effect of ensnaring strike.

I would just ask the DM if you can change the special effect of a spell. 

Based on Acid Splash:

You hurl stream of water. Choose one creature within range, or choose two creatures within range that are within 5 feet of each other. A target must succeed on a Dexterity saving throw or take 1d6 bludgeoning (or cold?) damage.

This spell’s damage increases by 1d6 when you reach 5th level (2d6), 11th level (3d6), and 17th level (4d6).

Based on Thorn Whip

You create a long, vine-like whip made of water that lashes out at your command toward a creature in range. Make a melee spell attack against the target. If the attack hits, the creature takes 1d6 piercing damage, and if the creature is Large or smaller, you pull the creature up to 10 feet closer to you.

The spell's damage increases by 1d6 when you reach 5th level (2d6), 11th level (3d6), and 17th level (4d6).

Easy Peasy no big deal.

Thanks for the suggestion but while my group isn't exactly against homebrew, we want to stick to the standard sets of spells, partially because we don't want to encourage too many homebrew stuff, but mostly because we kind of like the restrictions, it forces us to get a bit creative. (My cousin loves using mold earth, create water and prestidigitation to make a scented bath on every one of our long rests, which is pretty hilarious given how we just entered the tomb of the nine gods and we essentially have a hot tub goin in the middle of a golem-making room)

If you're avoiding homebrew, then shape water "doesn’t have enough force to cause damage" and you can only freeze water "provided that there are no creatures in it." So, short answer, no.

If you are homebrewing, then Wysperra's suggestion is the best one. Shape Water is not a combat spell, just reskin a different combat cantrip to use water/ice and change the damage type, rather than trying to balance an entirely new two-person combo combat use of a non-combat spell. There is no reason to suppose that Shape Water is the only way that a waterbender can manipulate that element as a cantrip, when it would be so easy and preferable to use Water Splash, Ice Bolt, Water Lure, etc...

Shape it into an icicle around a vial of acid or alchemists fire, catapult (spell) it into someone, do they break it off? disco inferno, does the ice shatter? burn baby burn. game effects up to the gm but it might give a penalty to putting out the fire or watering down the acid at least  considering the impact.

Start experimenting with weights of liquids do any contact poisons have a viscosity to avoid instant dilution and remain on the surface of water or ice you manipulated? use water to transport your toxin.

If you want water bender though with not too much reskinning catapult deals bludgeoning damage so ice ball and hurl is right up there, iceknife could work out for you as well although you may have to put up with being and ice bender rather than water.

One use icicle knives might be an option but probably no proficiency to hit as they are definitely not well crafted.

In the underdark, on finding green slime surround it with water, turn to ice and continue to shape and refreeze your slime cored ice sphere until you find an opponent that doesnt have fire,  bean it in the face and run away laughing as it melts from the slimy death (then get asked to leave your group by the dm who is tired of all this)

If you could use shape water to cause damage, the spell would say so. I'm glad you're  being creative but trying to invent a two character combo for which there is no rule is already a homebrew when you said you were trying to avoid homebrew. Best to talk to your DM and what they say. A good one will work with you to come up with options - maybe take some levels in Monk so you can use:

Water Whip

You can spend 2 ki points as an action to create a whip of water that shoves and pulls a creature to unbalance it. A creature that you can see that is within 30 feet of you must make a Dexterity saving throw. On a failed save, the creature takes 3d10 bludgeoning damage, plus an extra 1d10 bludgeoning damage for each additional ki point you spend, and you can either knock it prone or pull it up to 25 feet closer to you. On a successful save, the creature takes half as much damage, and you don’t pull it or knock it prone.

Catapult would be an option. 

I think homebrew would really be the best bet, but if the group is against homebrewing / reskinning abilities, I think it would be fair to bend the rules a little.

That being said, if you look at Waterbenders in the avatar, most of the time their attacks aren't necessary to deal raw damage like getting hit by a fireball or brick. If you had water impede other creatures movement, that would still feel a lot like waterbending and have some use to the party. 

 If the idea is to be like Avatar, then just take up Four Elements Monk. Done deal.

Quote from Wysperra >>

 If the idea is to be like Avatar, then just take up Four Elements Monk. Done deal.

 eh it was more of a spur of the moment thing, currently he's a 5 warlock 4 sorcerer and he started having a lot more fun with the shape water cantrip recently, especially after he got his hands on an Alchemy Jug (fun story, we convinced a tribe of vegepygmy's to trade their grung prisoner for a bucket of mayonnaise) and he's starting to try and take shape water as far as he can go (he froze water as he was pouring it out to slide on it ice-man style, and he has now taken to carrying around a floating blob of water around with him via shape-water, and i quote, "because i can") and now he's trying to figure out if there's some way to use it as a weapon, so far though his best use on that end is floating the water high above an enemy's head, turning it into a block of ice, and then just letting it drop, but that takes so many actions so he doesn't get to use it very often and he's trying to find faster ways of using it to fight.

Edit: just got an idea! he might not be able to use it since he doesn't really use ranged weapons (because duh, eldritch blast) but would it be too much to change the shape of the water into a crossbow bolt, freeze it, and then give it to someone who does use crossbows to use as ammo? one of our guys is a ranger/druid and they tend to run out of bolts fairly quickly for whatever reason so he has to keep gathering up the bolts from a fight and hope they aren't busted, but if we can make "ice bolts" then that might solve that problem, or at least alleviate it a bit.

You absolutely can use items fashioned out of ice with Shape Water as improvised weapons, making Tavern Brawler kind of a fun choice for a caster with access to the spell. But improvised weapon rules are pretty sketchy and open to DM interpretation, so take the time to talk about how they work with your DM before getting too invested in the concept. 

Using three actions just to shoot once with a crossbow is a giant waste of actions, but whatever floats your boat :)

The easiest thing to do here would be to make a 4 elements monk, would that work?

I don't see what's so bad about reskinning a spell in terms of trying to stay away from homebrew. The only complication I could see is damage types. If the spell is acid / lightning / frost its really easy to rp that the attack is water based, but some kind of acidic water, water with a lightning effect, or a  freezing water effect when you cast. If the damage type is fire... well yeah that's a little weird if you don't homebrew the damage type to something diffferent.

If you don't change the damage type the change is purely aesthetic to fit the flavor and style of the PC. It's the same spell, you're just describing it differently. Depending on the class and actual spell selection, sure, not every spell will be able to fit the water theme, but a lot of them will.

From the broadest spell list of the wizard, I see a ton of spells that could easily fit your water theme though.

Acid Splash, Frostbite, Minor illusion, Burning Hands (if you change the dmg and lose the extra affect of nearby flammable objects catch fire). Fog Cloud, Grease, ice knife, mage armor, magic missile, web. Those are just cantrips and 1st level spells. your friend could totally pull off the theme if they really want

I know this is a bit late but you can use shape water on rain above someones head to freeze it to cause hail, but that may not do too much. A second use could be to freeze water using it into ice spears and then launching them via the catapult spell. Seeing that they aren't too big you may be able to make multiple spears with a single casting.

Drain the water from an enemy's body. This is not official, sadly.

Quote from XorianAeravel >>

Drain the water from an enemy's body. This is not official, sadly.

 If you could do this with the spell it would say so.

The rules don't say everything, but I agree that this is unfair.

That'd be one nasty cantrip if you can use it to simply drain all the blood out of a target's body.