
Peter Fuesz |

In my game only 2 characters went into the submerged section. One had dark vision and the other had low-light and the everburning torch. Neither had spellbooks. Be sure to read the section on underwater combat in the DMG.
If you are worry about his spellbook just remind the player that water and books do not mix well, if he then still insists on heading into the water... does the ink on the pages run? Is his spell book water proof? I would warn him first tho.

ultrazen |
My own thought is that the special inks and such used in a spellbook make them reasonably weatherproof and smudgeproof. (I also consider scroll/map cases watertight.) There isn't anything "official" I'm aware of to back that up though. If you are really worried about it, provide a small, watertight bag or pouch of some sort for the spellbook. If the standard spell component pouch can be watertight, I don't see why it should be a big issue to protect a spellbook.