Diving Spider, Colossal (CR 11)

Colossal Vermin
Alignment: Always Neutral
Initiative: +2 (Dex); Senses: darkvision 60 ft., tremorsense 60 ft., and Spot +7*

AC: 22 (-8 size, +2 Dex, +18 natural), touch 4, flat-footed 20
Hit Dice: 32d8+64 (208 hp)
Fort +20, Ref +12, Will +10
Speed: 30 ft., climb 20 ft., swim 30 ft.
Space: 40 ft./30 ft.
Base Attack +24; Grapple +50
Attack: Bite +26 melee
Full Attack: Bite +26 melee
Damage: Bite 4d6+15 plus poison
Special Attacks/Actions: Poison
Abilities: Str 31, Dex 15, Con 14, Int -, Wis 10, Cha 2
Special Qualities: hold breath, Vermin
Feats: -
Skills: Climb +16, Hide -10*, Jump +10*, and Spot +7*
Advancement: 33-60 HD (Colossal)
Climate/Terrain: Temperate aquatic
Organization: Solitary
Treasure: 1/10 coins, 50% goods, 50% items

Source: Stormwrack

All diving spiders are aggressive predators.

Diving spiders have roughly circular bodies surrounded by outspread legs: 40 feet wide* and 10 feet high.
*The number includes the spider's body and its legs; the actual body diameter is about a third of the total.

Some breeds of diving spider are adapted for an aquatic existence. Rather than constructing webs, they build submerged, air-filled domes of spidersilk where they lie in wait for prey to wander too close.

Diving spiders spend their lives underwater, except for brief trips to the surface to replenish air. The spider's furry body holds air bubbles that it releases into the dome. They are expert swimmers, unlike most spiders.