If my PC has the shape water cantrip and a source of water within range can they create a limb to replace their missing one out of said watersource?

Talk to your DM. I could see it working, but kind of edge case.

I am not your DM, but I must admit, I would let this happen, just because it was a creative no game breaking way to handle a situation. Rule of Cool basically. lol

For an hour of watery amusement, I say it's not an option I had considered but seems plausible within the confines of the spell. Just be wary, if they decide to freeze the arm, so it's more solid, I don't believe it'll hold up well.

"Form into simple shapes" is what I see as the nub of the question. Is a prosthetic limb a "simple shape"?  I think a functional leg could definitely be considered a simple shape. It might not have individual toes etc. An arm is pretty simple.  A full five-fingered hand is pushing the limit of what I would consider "simple", but a mittened hand, or two fingers and a thumb should be doable, as would just wrapping the water around an object.  The cantrip lets you animate the water as you desire.  So. think making and animating a limb should be possible.

Attaching the new prosthetic to stump is a different question, but I would be inclined to allow that too.

Over all I can see it working, and it only lasts an hour. I would expect that when the spell ends the water in the limb would drain away and a different source of water would be needed to make a new one.

I figured I could carry a gallon jug of water on me that hung on that side and I drew the water out of to form the arm and even let it go back in when time was up. I would only use the arm when necessary and go about as a one armed adventurer for most of the time.

This puts the water source in my 5' cube.😏

To me, this fun harmless RP. I like it because it gives you a flaw for me to work with and something you can use to RP.

As a DM I love the idea, I hope your DM likes it too. Just remember, your DM gets final say.

How are you supposed to grip anything with a hand made of water?

Wow, that's almost philosophical.

I would say the arm would work as an aesthetic prosthesis, mostly. Much more than that would be too much like a super-hero situation for any D&D campaign that I would run. Besides, you know how it is with a lot of players. Give them a finger and they'll take the whole... well, you know.

I figure if I can freeze it I can almost freeze it. Making it just solid enough to equal the pressure that a meat tube could.

OR

I would allow this, it makes things interesting, I like novel ideas for things or using magic in RP ways.