Cleon
Legend
I have no idea!
Wikipedia says that raft spiders "can submerge altogether to hide from predators" but isn't very specific.
I think we're going to have to ask Cleon and demiurge about this.
Incidentally, looking back at the Stormwrack notes, I don't think they capture the diving bell spiders very well. We should probably redo those.
A raft spider does cover itself with a film of air so it can breathe underwater, although it doesn't build a "diving bell" air bubble web. In 3E terms I'd consider it a variation on Hold Breath.
I'm a bit dubious about the "break off a leg and use it as an aqualung" trick, but considering it's a matter of surface tension that shouldn't even work if the spider is enlarged to man-sized, I may be prepared to forgive it as some form of weird power.