Hi everyone! I'm a very, VERY green DM running a campaign at our local game store. My group is about to come up on an underwater puzzle. Basically, there's a crystal under the water. While it is in the water, it can be broken with bare hands. Out of the water it is nearly as hard as a diamond. The check to get it is going to be strength, but should it be at a disadvantage? Or just a very high DC? 

Another wrench in my particular gear is the fact that one of them is playing a Water Genasi, would the check effect him in the same way? Thanks for reading! Answers appreciated!

The rules for underwater combat specify disadvantage on attack rolls unless the character has a swim speed. It's plausible to apply the same to strength checks but it is not specified in the rules.

Gotcha, so everyone else would be at a disadvantage except for the Genasi. Thank you!

So, one quick question: when you say "get it", do you mean "to take it without breaking it" or to break it before it gets out of the water?

Asking because if you mean get it without breaking it, there is a potential wrinkle there, there you could add in terms of object damage...

So you have a crystal, and it's underwater. And while it's underwater it can be easily broken.  So if it's a strength DC, it'll be low., and there's no reason to have advantage/disadvantage on the strength test itself. Movement or dexterity I can see, but this is is more like a dead lift according to what you've given us.

Now, the bigger question is, what are they supposed to do with this crystal? if it's already underwater, then why would they bring it out of the water? is it stuck? What is the challenge?

It's a vital ingrediant for a medicine to a sickness that is killing off the local tribe!  The gimmick is, while it's easier to break underwater, it's still gonna take some oomph to get it snapped off. While it's above water, it's as hard as a diamond. It's like the difference between breaking a stick and trying to crack open the whole tree. 

It's a large crystal formation like a quartz cluster, the entire thing stretches to the bottom of the water, with some spikes being massive while others are smaller and thinner. A piece will need to be broken off, it's too big to carry all at once.

If something is as hard as a diamond to break i'd give it a hard or very hard DC to do so.

I don’t know you’d need to give disadvantage to break something off bare handed. The underwater disadvantage applies in the case of most weapons, but that’s largely because most weapons require the wielder to give a big swing. Thrusting weapons, like spears, don’t have disadvantage. 
But there’s no big swing involved in just grabbing something and breaking it. I might give the genasi advantage, but no penalty for the others. 
Also, since you say you’re new to DM’ing I have one other bit of advice. If getting this thing is plot critical, don’t put it behind a roll. The dice will screw you. Or, if it does go behind a roll, make sure you have a backup plan. 

Quote from ConspiretheTiefling >>

It's a vital ingrediant for a medicine to a sickness that is killing off the local tribe!  The gimmick is, while it's easier to break underwater, it's still gonna take some oomph to get it snapped off. While it's above water, it's as hard as a diamond. It's like the difference between breaking a stick and trying to crack open the whole tree. 

 Gotcha.  I wouldn't do disadvantage/advantage, just different DC's.  Disadvantage is a devastator. Especially when you roll extremes like me...lol