Hello mortals!

If you are going to venture into Watcher's Keep, I'd recommend bringing some potions of perception and master thievery. I have Nalia and Imoen in my party and their thieving skills aren't enough to disarm and unlock some of the encounters in WK.

If you choose to bite the bullet, bring some restoration scrolls if your divine caster doesn't have restoration. I'd say about 10 or so...some of the monsters have level drain abilities. Alternatively, you can use negative plane protection - just use it before hand. Jaheira is my only divine caster and she gets negative plane protection but not restoration.

Alternatively, if you are high enough in level to have access to Summon (Fallen) Planetar, you can use that spell, as the summoned Planetars can cast Lesser Restoration. There's also Wish, which (if the caster's Wisdom is high enough) can also grant your party a Restoration effect, but that's not reliable. (Note that if your caster is level drained below level 18 and is not a Wild Mage, she will lose the ability to use those spells, though having a Projected Image cast Wish (or Lesser Restoration, of course) from a scroll might still be an option.)

Nalia totally sucks as a thief anyway...on my first playthrough back in 2000 I only had her as a thief in my party...often had to use "Knock" for chests that are no problem for all the other thieves in the game (though the general lack of single-class thieves is one of the most annoying issues in BG2 in my opionion). Can't imagine anybody only using Nalia as a thief in ToB.
Don't recall any really bad problems with Imoen though...her thieving abilities are generally sufficient for the important stuff.

Which is why your spellcaster who needs to memorise spells should be wearing the Amulet of Power that you get from Aran Linvail. It conveys immunity to level drain.