Swarm Globes are a bluish-globes of glass a foot across, meant to be hurled by catapults, though other methods can be used. They weight 20 lbs, and are magical devices.
Each contains a swarm of creatures held in magical stasis and sometimes compressed in size as well, on breaking the globe, which happens on a hard impact, the swarm wakes up, reverts to original size if appropriate, and is very upset...and takes it's anger (and hunger) out on whoever is close. While they may not defeat an enemy they cause so much havoc they often let a ship flee as the enemy try to deal with the swarm.
-In useage, they're fired from catapults onto enemy ships as a normal attack.
To prevent accident, the globes are magical strong, as strong as iron, until a command word is spoken. This command word is always engraved on them in Common, after speaking it while touching the globe, the globe shimmers and after a few seconds when the shimmering ends, it becomes fragile as glass once more.
-In useage, the globe is loaded onto a catapult, when ready to fire, the artillery comamnder touches the globe, speaks the word, then immediatly fires it. Since the globe is still strong as iron on launching, it won't break, but will do so on striking the enemy ship or any hard object.
Typically a Swarm Globe contains a swarm of rats, centipedes, or locusts, though other creatures are possible, but at higher costs.
Swarm Globe
Moderate Alteration: CL 10; Craft Wonderous Item, Otiluke's Resilient Sphere, Polymorph. Cost: 2,500 gp. Weight 20 lbs.
1" thick glass (treat as iron until command word is spoken)
Also, it seems to me that pirates would have more use for nonlethal weapons than merchants. Afer all, it's in their best interest to avoid damaging the ship, cargo, or any wealthy passengers who might be held for ransom. Merchants under attack by pirates just want to stop the pirates by any means necessary.
Wyvern
it would be extremely effective on either side, really.
i mean, the distraction ability alone would be nice, for keeping crews down. the main problem i see (as Wyvern already touched on) is cost: it's great for a pirate crew to use as a one shot item because they are capturing something worth it, and an immobilised target is one that they can catch up to easily. furthermore, merchant ships are much less likely to have experienced warriors (read: people with more hit points to survive low level attacks, and to easily make the saving throw for distraction) as compared to a pirate ship (which is expecting combat, and gets into combat regularly).
it's much too expensive for a merchant ship to use regularly... particularly since you may need several of them to hit enough crew to make good your escape. (10 foot square vs 100 foot long ship? i have my doubts).
i will give my personal variation on the item, and see how you like it.
Swarm Globes: description as original.
costs:
celestial (or fiendish) spider swarm for 3 rounds (300 gp, price as single use, use activated item). requirements: craft wondrous item, summon monster II
celestial (or fiendish) bat or rat swarm for 5 rounds (750 gp) requirements: CWI, SM III
celestial (or fiendish) locust swarm for 7 rounds (750 gp or 1,400 gp) requirements: CWI, SM III or IV; depends. higher CR than bat and rat swarm, but no special abilities might lower it's value.
most of the summoned creatures seem to be (give or take) CR 1 below the spell level (though i only did quick checking at lower levels, to be honest... more research could be done). add the template so it can be summoned, and give it a duration, and there you have it... i could see a merchant blowing 300 gp for a single use spider swarm.
and so forth... essentially, price it according to a rough estimate of where the swarm would fit into a summon monster spell (even though you can't technically summon them, i would say it certainly fits).
and just for clarification, each globe summons a specific type of critter; the second kind doesn't give you more options, except options when you are creating it.
1) Price. Considering the expense a SJ ship is, and how much a good merhcant could make...or lose...2,500 gp is not much lol, and it's in keeping with prices as per the DMG, which is also important for game balance.
2) You don't want anything TOO lethal in the globe, just in case of accident, also very lethal things = more expense.
A SUmmon Swarm spell exists iirc, also, be kind of easy to capture a lot of vermin aorund docks for use, so not only does it give a cheap bio-wepaon, you also rid a harbour of rats etc ;)
3) best creatures would be wasps or the hornets, but those aren't in the Monster Manual, and Hellswarm waps are WAY too nasty.
4) yeah the idea is to so screw up the enemy, they can't pursue as you flee. Swamrs are nto the best weapons for pirates becuase they'd have to remove the vermin and suffer as wlel, but they are a possibility of course as an offensive weapon, especially if you have ways of neutralizing or removing them.
5) It's a simple weapon, which is important, as most crews are not gonna be adventurers with a life time of sneaky dirty tricks and smarts.
6) Game balance, you don't want this at it's cost to be small item, or you'd lob many of these at enemies, and have them lob them at you! Making it a 1 foot, 20lb sphere with beings in stasis inside keeps it nice for traps and catapult/giant thrown.
If it was like a Bead of Force and summoned swarms, owie! that would be brutal, and would have to be more expensive.
7) A bale of hay would stop burning in the void, in the Phlogiston you'd blow yourself to hell, also in the Phlogiston SUmmons pelsl wouldn't work so a sphere that used summon monster wouldn't work.
Fire is dangerous as hell, you may set the enemy on fire...or yourself.
8) Swarms would put most folk on deck out of action in the swarm area, so they couldn't cast, couldn't use ship wepaons, and if the swarm got inside, maybe the helmsman would suffer.
the whip is the only one that is single use, all the others are reusable more or less indefinately. and it costs 500 gp, has an attack bonus of +10, improved grapples at +15, can fly, counts as a magic weapon, and lasts for up to one hour.
(actually, on closer inspection, you also have the horn of valhalla... but again, it's reusable. there's also the pipes of the sewers, but again, they're reusable. and dirt cheap. and they actually summon multiple rat swarms. then there's the robe of bones, which summons 11 HD worth of undead, for 2,400 gp. and the various items to summon elementals... again, reusable. the only guideline for a single use, 'summoned creature' type effect is the whip, really... and given a choice between 1 swarm your way or 5 whips, i as a ship captain wouldn't even have to think about it before taking the whips.)
as far as difficulty removing them, well, swarms are not tough to get rid of... provided you know in advance that you need to be ready for a swarm. torches in the hands of 4 or 5 decent warriors should be enough for most low CR swarms.
and once again, don't think of this as being too likely to end an attack on you. a 40 ton ship is going to be 4000 cubic yards... that's up to 40 yards by 10 yards by 10 yards, or 120 feet by 30 feet by 30 feet... one 10 foot square is not covering a very large portion of the deck, really. you are going to *need* multiple swarms to get any decent coverage.
seriously, this is not significantly more powerful than a summon monster spell, if at all. and furthermore, you don't even control the swarm. flavor it however you like it, but if it's as powerful as a summon monster spell, you should price it the same way.