Hey all! So, currently in the process of trying to homebrew some water-based spells, and am having a lil' bit of trouble.

So, most of the spells I've written take inspiration from water-based actions that other monsters have (like a marid's Water Jet, etc), and rewording these for player usage has been fairly easy. However, I am struggling with rewording this action from the nereid:

"Water Lash. The nereid causes a 5-foot cube of water within 60 feet of it to take a shape of its choice and strike one target it can see within 5 feet of that water. The target must make a DC 13 Strength saving throw. On a failed save, it takes 17 (4d6 + 3) bludgeoning damage, and if it is a Large or smaller creature, it is pushed up to 15 feet in a straight line or is knocked prone (nereid’s choice). On a successful save, the target takes half as much damage and isn’t pushed or knocked prone."

It's been reworded to:

"You cause a 5-foot cube of water you can see within range to take a shape of your choice and strike one creature you can see within 5 feet of that water. The target must make a Strength saving throw. On a failed save, the target takes 4d6 bludgeoning damage, and if it is a Large or smaller creature, you can push it up to 15 feet in a straight line or knock it prone. On a successful save, the target takes half as much damage and isn’t pushed or knocked prone."

So, the wording there is okay (If I may say so myself). However, I want to make the spell a 2nd-level spell, since there isn't really that much choice for water-based 2nd-level spells, and I figured the best way to do that would be to reduce the damage slightly, and allow the caster to repeat the water lash over the course of the spell (upping the duration to Concentration, 1 minute). 

So basically, my trouble is coming from trying to word the spell so that the caster can affect the same cube of water, or a different one, and use it to strike the same creature, or another one, without making it too complex and difficult to understand.

tl;dr: Need help with wording a spell so that it can be used to strike multiple times, without making it too wordy.

Any help would be much appreciated!!

Until this spell ends, as an action you can cause a 5-foot cube of water you can see within range to take a shape of your choice and strike one creature you can see within 5 feet of that water. The target must make a Strength saving throw. On a failed save, the target takes 2d8 bludgeoning damage, and if it is a Large or smaller creature, you can push it up to 15 feet in a straight line or knock it prone. On a successful save, the target takes half as much damage and isn’t pushed or knocked prone.

How’s that? All I added/changed was the highlighted bit.

Thanks for the quick response! The first solution I had was similar; changing the first line to:

"When you cast the spell, and again as an action on each of your turns for the duration, you can cause a 5-foot cube of water that you can see within range to take a shape of your choice and strike one creature you can see within 5 feet of that water."

But I just worry that it might be too vague? I'm not entirely sure :/

I just wanna keep it simple to understand, without any ambiguity in being able to repeat the attack.

Okay, then:

 Until this spell ends, as an action on each of your turns you can cause a 5-foot cube of water you can see within range to take a shape of your choice and strike one creature you can see within 5 feet of that water. The target must make a Strength saving throw. On a failed save, the target takes 2d8 bludgeoning damage, and if it is a Large or smaller creature, you can push it up to 15 feet in a straight line or knock it prone. On a successful save, the target takes half as much damage and isn’t pushed or knocked prone.