Threat: Sea Swarms

Our game takes place in a coastal area. One of the fronts has to do with the rising sea. I've got this threat:

Is Called: The Things of the Deep
Kind: Brutes
Impulse: Hunting Pack - To victimize anyone vulnerable
Description & Cast:
I want to add a whole bunch of sea monsters eventually, I have provisions for what starts showing up when in my flood countdown. For now it's only these guys:
Sea Swarm (2 harm, pack, aquatic): Translucent round jellyfish things that move in frothing packs and suck the blood from their victims. Can leap from the water. Attracted to blood. Not fooled by standing still.
Custom Moves:

 When you roll harm for being stung by sea swarm tentacles, if you roll 7-9, add "You feel faint from blood loss. Take -1 forward" to the list of options the MC can choose from. If you roll 10+, also add "Your flesh is badly stung. Take -1 ongoing until the nematocysts are removed."

When you use an angel kit to remove nematocysts, roll +Sharp. On a 10+, they're fine, though it'll leave a nasty scar and be sensitive for a week. On 7-9, choose 1:

• The stings are in deep, and it costs 1 stock to surgically remove them.
• The stings start pumping blood. Your patient takes 1 harm.

On a miss, all of the above, plus the wound's infected.

Anyway, how's that sound? This is my first time doing this.   

Sounds pretty solid! A good guideline to follow for custom rules, which I believe I've seen Vincent allude to cryptically across the threads, is if they are adding to play and making things interesting, great! Keep the rule in play! If they're starting to hinder the fiction or the options aren't adding much to the game, ditch the rule or tweak it as necessary.

Any reason the custom move has you roll+sharp instead of roll+stock spent as per the normal angel kit rules? Would this require a second roll using the normal rules to actually heal any harm? I like that on a 7–9 the Angel has to choose to spend a stock or rip them out the old fashioned way.

It's certainly horrifying - a frothy wave of leaping vampiric jellyfish.  Probably beautiful, too.  I wonder how you foresee this threat building and hitting.  When would you say "vampiric jellyfish leap out of the sea?"  And what do you figure the characters can do about it besides scream?  Would they rush to raise nets, pilot their boat out of the swarm before they're overwhelmed, or blow jellyfish off each other's necks with guns?  What do you figure will be the drama?

+Sharp because I see this as being more about the Angel using a steady hand and a keen eye to carefully remove the sting without having to actually spend stock. It's supposed to be closer to the 3:00/6:00 angel kit move in terms of severity, I guess. I don't know, that may or may not make any sense.
I guess it's true that if the patient already took 2-harm from the initial attack, and then the Angel gets 7-9 and picks the 1-harm option, the patient's at at least 9:00 and the Angel will probably be spending stock to heal them anyway. So it's a choice between 1-harm and spend stock, or spend 1 stock. Not much of a choice.
Then again, maybe that's okay - under most circumstances, the spend 1 stock is the obvious choice. If the patient had armor, or if the Angel has a reason to not want to treat their other wounds, or something of the sort, then the 1-harm makes sense as a possible option.

Ten of Swords, these things actually came up in-game already before I made them a threat. Generally I Announce Future Badness in the form of saying there's something frothing in the water off in the distance, and it's coming closer. The Gunlugger used a grenade to blow up the swarm that attacked him, and the Chopper had his whole gang concentrate their fire into the water when they faced this threat. We don't have boats in this game so far - the city's in the middle of a flood, so the streets are filled with water. (I hope we do get boats, though!) 

The Angel could also use it as leverage in manipulate. "I don't know if I have the stock to spare to take these out proper. Maybe I'll just rip them out.."

Seems like the moves are cool in play! I only asked because I was curious about your thought process. Thanks for sharing!

Very cool. I like this.

Just one 'rules lawyer' point. You might want to explicitly state that pulling them out bypasses armour, by adding the ap tag onto the harm. i.e.

* The stings start pumping blood. Your patient takes 1 harm (ap).