GT-54 with rain cover
 Dolphin 32, fully waterproof
Who cares about water?

T-28 with secure
protective rain cover
Waterproof Systems - Facts and Myths
Defining "Waterproof" In The Bike Bag Market - How Arkel's Waterproof Systems Work
Nearly all panniers on the market, with the exception of a few like our Dolphin's, are not watertight. This has nothing to do with the fabric, Arkel uses a watertight fabric that's also coated to repel liquid. It's the stitching on any pannier that creates tiny holes in the fabric and zipper closure contacts points where leaking can occur.
For most usage, this is not a problem. Arkel's panniers will easily resist a regular rain for an hour or two before experiencing any sort of seeping, which is more than enough for most cyclists to get at their destination. If the riding dictates more time spent in the rain, then Arkel's rain covers, our waterproof inner liners and dry bags inside the panniers will offer up to three layers of waterproof protection. The most layered protection on the market
But why not have totally watertight panniers to begin with, never needing to think about inner bags or covers?  
Watertightness: An Essay
If a bike can be loaded with gear and ridden in the rain, why settle for anything less than fully waterproof/watertight panniers?
The answer: Be careful what you wish for.
Creating a Yellow Submarine with a watertight pannier seems like it would be a joy, insurance that the rider's belongings are safely protected against the elements. A top loading dry bag must be the best choice.
Sometimes it isn't so obvious.
For more extensive riding we prefer a regular pannier that incorporates layered protection. Are we nuts? Read on...
Ask experience users of watertight panniers and they will tell you about battling condensation build up and that they use plastic bags inside the panniers to separate wet stuff from dry and to have some organization system that top load, one compartment bags don't provide. Having stuff separated inside make searching easier as these panniers are always of the one compartment top-loading type. Compartments are a precaution against the elements when digging through for something in a rainstorm and to avoid a possible water spill - in which case the open design watertight pannier becomes a watertight swimming pool with your stuff floating around inside.
Advantages of Arkel's more user-friendly, compartment and layered protection panniers? Our layer protection panniers can't hold water in, so we don't worry about it getting in to start with, pooling up, or staying in to create a condensation factory. Having extra compartments outside allows riders to store wallets, jackets and other often used items without exposing the main compartment to rain.
Sometimes cyclists fall. Not often, but it can and does happen.
Even at a stand-still, a fall on a bad surface can rip a pannier, any pannier. In one instant a watertight pannier becomes a useless pierced plastic bag. Bringing on the dreaded pool effect again. We've seen people voluntarily piercing draining holes inside their not-watertight-anymore-ripped panniers to fend off any accumulation of water at the bottom. Look, canoe and kayakers use watertight bags, but hikers and bikers use layered protection bags... why? Aside from the obvious watery nature of canoeing, we know that bags in boats aren't subjected to being ripped in the same fashion as when on a bike. Plus, the river is flat. A mountain, when hiking or biking, is not. When a bike goes down the bags need to be durable and waterproof, even with a tear. Arkel's are.
Barometric pressure causes rain, and rain is condensation in a form. Any hiker or biker knows that at one moment he can be comfortable and dry in his gore-tex jacket, but the next moment he can suddenly find himself drenched in inner condensation caused by a changed in barometric pressure, either due to altitude or a front coming in. The same principle affects any watertight bags. Here, air itself is the enemy.
If, after the noon lunch when you close the watertight bag, you are in a warm and humid high pressure system, but during the afternoon you ride in a cold, low pressure front, the humidity inside the panniers will condense as surely as death and taxes.
We love the convenience and safety of a front loading pannier with individual compartments - everything protected against road grit with a rain cover and everything more accessible when we need it, with less exposure to the elements. Our own dry bag series, the Dolphin 48-32 line, while a top loader, has inner waterproof liners and outside drains for wet clothing to stay free from creating condensation... and separate pockets to store electronic and wallets away from the main compartment. It's the most functional dry bag on the market.
In real life, everything has an up and downside, but with Arkel we have really studied how weather effects riders in the field, and over time we've designed and refined the very best in accessibility, function and the widest spectrum of choices in waterproof protection - the layered system and the dry bag plus series.