Dry Bag

From CanyonWiki

Dry bags keep separate your wet stuff from your dry stuff. During the day they keep the dry stuff dry. At the end of the day you can toss the wet stuff in to keep it from making everything else wet. Dry bags also can serve as floatation devices in your pack if they have a bit of air in them. Not that you'd want to be a sea with one, but it will keep the pack from sinking to the bottom of that nasty pothole.

Like all gear, there is a vast range from the cheap jerry-rigged to the expensive.

[edit] Garbage bags

These fall on the low end of the price spectrum, but often work just fine if they're tied up right, and getting your stuff wet is OK. Buy the trash-compactor type bags which are quite sturdy.

Advantages:

Cheap - although the more expensive double-ply are a little more secure.
Malleable.
Can be purchased en route at any convenience store.
Disposable

Disadvantages:

Can easily rip if not stored with care.
Not environmentally friendly.
Opaque
Fail often - your stuff gets wet

[edit] Professional dry bags

These come in a range of quality and price. As always, the more you pay, the more likely the gear is to work and to last. Intermediate grade quality will protect your fleece from short, intermittent dunkings. If you'll be swimming all day, fork out the money for a more expensive bag.

Advantages:

More durable than a trash bag.
Less likely to rip than a trash bag.
Look much cooler than a trash bag.
Some come in plastic so you can see all your cool gear inside.
Moderately malleable.

Disadvantages:

More expensive.

[edit] Keg

Recently, the keg has entered the canyoneering gear world. A keg is a stiff, watertight container that can protect fragile objects (eg your camera). They can be jerry-rigged with an old ice-cream container or more professionally purchased. These are popular in Europe, and can be purchased in the USA from Imlay Canyon Gear and better canyoning shops.

Advantages: