In that case, it's time to set the spider-sense tingling!
Raft/Swamp Spider
FREQUENCY: Uncommon
NO. APPEARING: 1-6
ARMOR CLASS: 6
MOVE: 18”
HIT DICE: 2+2
% IN LAIR: 50%
TREASURE TYPE: J-N, Q
NO. OF ATTACKS: 1
DMG/ATTACK: 1-6
SPECIAL ATT.: See text
SPECIAL DEF.: See text
MAGIC RES.: Standard
INTELLIGENCE: Animal
ALIGNMENT: Neutral
SIZE: M
PSIONIC ABILITY: Nil
The raft or swamp spider is found along lake shores and in marsh areas. She makes her den by the water’s shore and waits for prey to appear on or beneath the surface of the water. She then scurries across the surface of the water, skimming over it with the aid of the liquid’s surface tension. Trailing behind her is a safety cable anchored to shore so she can pull herself back in should that prove necessary.
When threatened, the swamp spider can hide underwater by walking down along the side of the marsh or on the lake bottom. The many hairs on her body and legs capture and hold air bubbles sufficient for from 10 to 12 hours or more of breathing. Also, the air bubbles provide buoyancy sufficient to bring her back to the surface when she wishes.
Originally appeared in Dragon Magazine #67 (1982).
I'm content to just stat 'em as Medium, with advancement to a few larger sizes, like some of the variant giant spiders that popped up later in 3.5 publications.