It looks like Stormwrack has suggestions for building diving spiders...
Diving Spider, Monstrous
Some breeds of monstrous 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.
A monstrous diving spider has the same statistics as a typical monstrous spider, with the following adjustments:
Speed: A monstrous diving spider gains a swim speed of 30 feet.
Special Attacks: A monstrous diving spider retains all the special attacks of the typical monstrous spider, with one change.
Web: Monstrous diving spiders cannot throw a web net, nor do they create sheets of sticky webbing.
Special Qualities: A monstrous diving spider retains all the special qualities of the typical monstrous spider, with the following adjustment.
Tremorsense (Ex): A diving spider's tremorsense allows it to detect and pinpoint creatures in the water as easily as those on land.
Hold Breath (Ex): A diving spider can hold its breath for a number of rounds equal to 6 times its Constitution score before it risks drowning. For a typical diving spider (Medium or Large), this is 48 rounds, or almost 5 minutes.
Skills: A diving spider has a +8 racial bonus on any Swim check to perform some special action or avoid a hazard. It can always choose to take 10 on a Swim check, even if distracted or endangered. It can use the run action while swimming, provided it swims in a straight line.
Environment: Temperate aquatic.
Shall we do our own version, or allow these to suffice for the diving bell version?