1) Can you climb up-side-down along the bottom to the other side of the boat? Obviously you could if the boat was in midair, but can you still climb while you are underwater?
2) If you can do #1 then how fast would the boat need to move so you got swept from under the boat by the water pressure? A strength check perhaps?
Interesting questions. Let's see...
From a RAW perspective, the only relevant part is this:
[...] need not make Climb checks to traverse a vertical or horizontal surface (even upside down).
No matter how difficult the check would normally become (e.g +5 for being slippery), you don't have to make one. Ever.
So the RAW answers would be: 1) Yes. 2) You wouldn't.
If you want to look outside of the RAW area, here are some things to consider:
Climbing underwater is actually easier than climbing outside of it, because of the buoyancy.
If the spider climb spell effect works similar to real world spiders' climbing ability, its function doesn't change underwater. It's no glue, but an electromagnetic force effect caused by myriads of tiny chitin "hairs". For spiders, the effect is 20 times stronger than needed, which means that they could easily hang beneath a ceiling by a single "paw". If your magic effect is similar in function, but present over your entire body, it can even be several additional magnitudes stronger.
For the second question, if you want to ignore the "no check needed" part of the spell, you could use the wind effects table as a guideline.
Considering the higher density of water on the one hand and its streaming behavior, your ability to press yourself flat against the hull of the ship to minimize your flow resistance and the strength of the effect holding you in place on the other hand... You'd need a really fast boat. Magically propelled, because the fastest mundane ship in the srd only reaches about 20mph. And even then, the TN for the strength check given in the weather table is only 15.
Thanks!
If you consider the underside of a submerged boat a slippery surface, the Slippers of Spider climb would not help you.
"Severely slippery surfaces—icy, oiled, or greased surfaces—make these slippers useless."
If you consider the underside of a submerged boat a slippery surface, the Slippers of Spider climb would not help you.
"Severely slippery surfaces—icy, oiled, or greased surfaces—make these slippers useless."
It wouldn't be slippery. If anything it would be a jagged surface due to barnacles.