Pearl of power is a no brainer.
A Necklace of Adaptation. Covers you in a buble of air, which makes it better than water breathing, since you can still cast spells while immersed. It also stops the effects of gasses. Saves your equipment from above effects, too, so no lost spell book if you get dunked. I'm still wearing one at 16th level.
Ring of Climbing/Jumping/Swimming. Cheap, and it saves you on having to put skill points there. You will need all eventually, especially swimming.
Wand of True Strike. Cheap, you need the bonus if you have to fight, and your local rouges may find them useful as well.
Cloak of Resistance.
Bracers of Armor.
Ring of Protection.
Don't know if you could afford it, but a Scarab of Protection.
My best advice would be to get the little things that make up for your weaknesses, but you never seem to have the time to find or make. But definatly the Necklace of Adaptation.