Waterproof, waterproof, waterproof - or do you want to pack in plastic bags?
I'll admit it, I'm a bit of a nut when it comes to protecting my gear - especially when I'm carrying electronic gadgets like a handheld PC and a digital camera. Packing in plastic bags works, but why should you need to deal with all of those bags? I highly recommend packing in waterproof panniers or duffles.
There are some really good waterproof panniers out there. And if you're using a trailer that uses a duffle, either make sure that the trailer comes with a waterproof bag, or purchase one separately.
I've ridden in driving rain with both of my set-ups (panniers, and trailer), and I've never had any water leak into my gear - and I've never packed in plastic bags.
I've heard some people say they wouldn't use waterproof panniers because if you put something wet in them it would just stay wet. That seems pretty silly to me! If you put something wet in either a waterproof or non-waterproof pannier, it's going to get other things wet... A similar argument I've heard is that if you're biking in a wet place like the Pacific Northwest that there's no way you'll keep water out of your panniers. Not true - I've ridden during some very wet stretches of weather, and I've never had that problem. Plus - if you need a bucket of water for some reason, you can always take your gear out of your waterproof pannier and use it as a bucket. OK, OK, that's a bit strange, but I've actually done it once. But back to the recommendation - I still recommend using waterproof panniers, duffles, or trailers!
I highly recommend Ortlieb panniers or duffles. I've used their Bike Packer series of panniers - waterproof while riding, but I wouldn't throw them into a lake. If you want the freedom to drop your panniers in a river or lake, you should probably use their "roller" series of panniers.
I've only used products from Ortlieb (waterproof) and Arkel (panniers & packs have an integrated rain cover. There are additional manufacturers who make waterproof panniers. I've opted not to list them here because available products change too often. Remember, Google search is your friend.
Would you like to make your own waterproof pannier's? Take a look at Brian Huntley's article Bike Buckets - An inexpensive pannier system you can make.
My Bike Friday suitcase trailer is also waterproof - no problem there.
That leaves just one more thing. For camping trips with panniers, what do you do with your sleeping bag? Some folks seem to manage with less gear than I do, and they stuff their sleeping bag into one of their waterproof panniers. Nice solution, if you can pull it off. I couldn't - so I stuffed my down sleeping back into a Hydroseal stuff sack from Outdoor Research. This is stuff sack that is waterproof and has a roll-down dry-bag style closure. No leaks there either.
Panniers, a trailer, both?
For the touring cyclist... how much did you say you need with you
And where are you going to put it?
For the touring cyclist... how much did you say you need with you
And where are you going to put it?