The mermen are large creatures, while the sharkmen are medium creatures. I should have described them in more detail. Okay, I've edited my post a bit.
If you target a point just outside the circle of baddies, you could potentially hit two of the mermen without endangering the hostages. The two weresharks are closer to the hostages so that's a bit more tricky. You can't get both sharkmen, but you can get one sharkman and a merman. Does that make sense?
To complicate matters, I believe Zale is right there underneath it all, and you can't see him, and he has no idea you're bringing a lightning storm down on top of him. So if you cast, I will randomly determine via die roll whether he also needs to roll saving throws vs. your spell.
TL;DR: You can do it without endangering the hostages, but with some risk to Zale.
3rd-level conjuration
Casting Time: 1 action
Range: 120 feet
Components: V S
Duration: Concentration, up to 10 minutes
A 10-foot tall, 60-foot radius storm cloud cylinder appears, centred on a point you can see within range directly above you. The spell fails if your location can't accommodate the cloud's size or height. When you cast the spell, choose a point under the cloud for lightning to strike. Each creature within 5 feet must make a Dexterity save, taking 3d10 lightning damage on a fail and half on a success. While the spell persists, you can use your action to call lightning to a point in range again.
If you're outdoors and in a storm, the spell gives you control of the storm rather than creating a new one. The lightning damage also increases by 1d10.
At Higher Levels: The damage increases by 1d10 for each slot level above 3rd.