In a recent gaming session, I had a person playing a druid who wanted to use Shape Water to create a weapon (specifically a scimitar).  His argument was that magic could sufficiently harden the water to make the weapon viable.  In the end, I ruled it as an improvised weapon and let combat proceed.  But I'm concerned this wasn't the right approach, and that perhaps I've set a dangerous precedent for the misuse of such spells.

The second spell I'd like opinions on is Ceremony (the 1st level Cleric spell from Xanathar's Guide To Everything, pg. 151).  In the same group, I have a person playing a paladin, and the group got themselves into a situation where diplomacy had failed and combat was going to be inevitable.  The paladin stated she'd be uncomfortable engaging in the fight, but didn't want to sit it out as she'd fall further behind the others in experience.  She sought out a cleric of her deity, and off the cuff I decided to add an option to Ceremony called absolution.  I made it so by receiving this absolution, any actions the paladin takes in the next 12 hours are pre-forgiven by their deity, but I put the qualifier that this doesn't prevent against unwilling alignment changes due to magic or other effects.

I'd love to hear the opinions of other DMs on these, and perhaps any tips on how to better be prepared for surprise situations like this.

Here's some sage advice:

"Spells officially do only what's described in their text. However, players often come up with creative, alternative uses, which DMs are responsible for adjudicating. I encourage a DM to say yes to such uses when they're non-disruptive and fun."

"I encourage players to reskin classes/subclasses/backgrounds liberally. Take the class/subclass/background that fits the function you want for a character, and then tell your character's unique story."

What the player attempted to do with Shape Water is well outside the scope of the spell. However, letting them create a scimitar would be harmless as long as you don't consider it to be a magic weapon. If that's what'll make them happy, let them have it. You don't have to say yes to future requests to go outside the scope of the spell. The water scimitar could just be this one extra gimmick their character learned.

Likewise, if you want to add more uses to the Ceremony spell (which is already mostly there for roleplaying), that's fine. Personally, I don't think the Paladin needed forgiveness if they tried diplomacy first, they weren't the aggressors and they didn't kill anyone.

Shape Water can make ice sculptures that don't melt right away. I'd say an improvised weapon is a totally reasonable application. 

How effective a weapon made of ice is going to be depends on the kind of game you are running so it's really up to you. 

For me, I don't buy that the spell makes the ice extra strong or durable (just makes it not melt for the duration) so I'd have the weapon do 1d4 damage. Also maybe the weapon is fragile and may break easily, perhaps on a 1 to hit, and breaks automatically if successful targeted and hit by an enemy attack.

An ice dagger or club seem pretty viable,  swords and anything with a long handle or shaft, I might have the weapon shatter after the first time it deals damage. But that's just how I'd do it, again, depends on your play style.

For ceremony, I'd suggest giving all character the same experience points so everyone levels up at the same time no matter what rather than only based on participation in an encounter. Right there you solve the underlying problem which is that your player cant role play the character the way they want to in order to get xp.

Again, your game, your rules so the addition is fine if it works for you. I don't like it personally. What kind of good aligned deity would (and for a very small cost) pre forgive 12 hours of potentially unspeakable acts? And if it's not a good aligned diety, what are you asking forgiveness for anyway, right?

Thank you both very much for the responses.  They are very helpful, especially concerning Ceremony.  I've decided to take a middle approach - if this should come up again, I'll just have the cleric explain that since she honestly tried to avoid the combat, no sin accrues to her if she then has to, as long as she doesn't do something horrendous and/or dishonorable.

Mael, the problem is that the group I game with (or the majority thereof) demanded that we do individual experience, so that they can gain experience from doing class/subclass specific acts.  Such as rogues getting small amounts of experience for picking locks or disarming traps, spellcasters for casting non-combat spells, etc.  Unfortunately, this leaves a lot of the fighter-based classes lacking, since their abilities don't seem to translate well into acts I can award experience for.  Maybe that's just a lack of imagination or experience on my part.

Yeah, if the group wants to do xp that way and that's what's agreed to your are stuck there. I played in a similar type of game last year, with xp given out for successful skill checks. I really disliked it. I played a wizard and had very few opportunities for skill checks. I never got any xp awards for creative spell use or role playing, speaking to the DM did not help. The group was always wanting to roll dice to get xp, I think it killed a lot of role playing opportunities.

So when I started dming my current game I abandoned xp all together. The group levels up every 3 to 5 sessions depending on what's been happening. We don't miss xp at all.