Option 1: The spellbook is saved.
A. Yes, they can dry it out. The spellbook becomes a useful tool for the wizard to copy from.
B. The spellbook was enchanted to resist water, not only can the wizard copy the spells, but now they have a treasure in the spellbook itself.
Option 2: The spellbook is damaged.
A. The spellbook is now a soggy mess, not even worth the paper it was printed on.
B. The spellbook is damaged, but can be partially saved and used as a tool to springboard spell research in lieu of simply copying them.
Option 3: It's magic, yo! The spellbook has been corrupted by the seawater reacting to the magic ink and rituals, and the very odd circumstances in which they were introduced to each-other.
A. It is now something totally different than a normal spellbook. Perhaps it has all new water themed spells, has the power to summon water elementals by pulling out pages and expending them, maybe even form a portal to the elemental plane of water, or be used as a one-shot tool for creating a decanter of endless water.
b. It's cursed, much like above, but with dire consequences that may not be apparent at first. You know, drawing the water-logged remains of sailors who were lost at sea, back to land in order for the restless dead to return the book to the briny deep like a librarian with a vengeance.