This should be fairly easy. Take a (Small) giant leech, reduce it to Diminutive, drop Aquatic subtype, modify slightly.
I'd make it Tiny rather than Diminutive, but agree they look pretty easy.
I think we'd better give it a swim speed, since it lives in swamps.
I'll drop their Dex to 10, just to make them a bit more different from a leech.
Not sure whether to include the salt trick.
Scum Creeper
Tiny Vermin
Hit Dice: 1d8 (4 hp)
Initiative: +0
Speed: 10 ft. (2 squares), climb 10 ft., swim 10 ft.
Armor Class: 12 (+2 size), touch 12, flat-footed 12
Base Attack/Grapple: +0/+2
Attack: Bite +2 melee (1)
Full Attack: Bite +2 melee (1)
Space/Reach: 2½ ft./0 ft.
Special Attacks: Attach,
facehugging, gnaw
Special Qualities: Blindsight 10 ft.,
blind or myopic, salt vulnerability, scent, wallcrawling
Saves: Fort +2, Ref +0, Will -1
Abilities: Str 6, Dex 10, Con 11, Int --, Wis 8, Cha 2
Skills: Climb +8, Hide +12, Swim +8
Feats: Weapon Finesse (B)
Environment: Warm marsh or underground
Organization: Colony (2-4) or swarm (6-11)
Challenge Rating: 1/6?
Treasure: None
Alignment: Always neutral
Advancement: 2-3 HD (Small)
Level Adjustment: ---
Attach (Ex): If a scum creeper hits with a bite attack, it uses its myriad legs and suckers to latch onto the opponent's body and begins to use its gnaw attack (see below)
, if its bite attack scores a critical hit it also uses its attack (see below). An attached scum creeper is effectively grappling its prey and gets a +12 racial bonus on grapple checks (already figured into the Base Attack/Grapple entry above).
An attached scum creeper can be struck with a weapon or grappled itself. To remove an attached scum creeper through grappling, the opponent must achieve a pin against the scum creeper.
Gnaw (Ex): A scum creeper automatically does 1 point of damage in any round when it begins its turn attached to a victim.
Salt Vulnerability (Ex): A handful or more of salt deals 1d4+1 points of damage to a scum creeper on contact and causes it to detach itself from its victim.
Wallcrawling (Ex): A scum creeper need not make Climb checks to traverse a vertical or horizontal surface (even upside down). It retains its Dexterity bonus to Armor Class while climbing and opponents get no special bonus to their attacks against it.
Skills: A scum creeper has a +4 racial bonus on Hide checks and a +8 racial bonus on Climb and Swim checks. A scum creeper can always choose to take 10 on Climb and Swim checks, even if rushed or threatened. A scum creeper uses its Dexterity modifier instead of its Strength modifier for Climb and Swim checks. It can use the run action while swimming, provided it swims in a straight line.