Here are the monstrous crab stats from Stormwrack...
Monstrous Crab, Small
		
		
	 
Reduce that to Tiny and cut the Toughness feat and it'll do for the component animal of a Crab Swarm...
Monstrous Crab, Tiny
 Tiny Vermin (Aquatic) 
Hit Dice: ½d8 (2 hp) 
Initiative: +0
Speed: 20 ft. (4 squares)
Armor Class: 16 (+2 size, +4 natural), touch 12, flat-footed 16
Base Attack/Grapple: +0/-6
Attack: Claw +2 melee (1d3-2) 
Full Attack: 2 claws +2 melee (1d3-2) 
Space/Reach: 2½ ft./0 ft.
Special Attacks: Constrict 2d3-2, improved grab 
Special Qualities: Amphibious, low-light vision, scent, vermin traits 
Saves: Fort +2, Ref +0, Will +0 
Abilities: Str 6, Dex 11, Con 10, Int —, Wis 11, Cha 2 
Skills: Hide +12, Spot +4 
Feats:  		—
Environment: Temperate aquatic 
Organization: Colony (8–16) [
?]
[
I like Solitary, scrabble (2-4 swarms), or horde (5-8 swarms) for the Swarms]
Challenge Rating: 1/4 [
I feel it should be lower, like CR 1/6 or 1/8]
Treasure: None 
Alignment: Always neutral 
Advancement: —
Level Adjustment:  		—