What wonderful photos! A fantastic, wonderful Thorn with a neat load and a Click-Stand; you've managed to capture three of the nicest things that appeal to me in a bicycle photo. Add in the scenery, and it makes me want to see many more in our gallery. I hope you'll add some for us to enjoy.
Seals are meant to keep dirt and most water out?
...and I am inclined to think they do, to a degree. Anything helps. I do believe well-greased hubs (and whatever they use for shielding) work better than the alternative. When I started commuting regularly in the late-1970s (when dinosaurs roamed the earth), my Shimano 600 hubs were completely unshielded, had large clearances between the dustcap and hub cone, and the grease would break down (emulsify) with exposure to rain and curbside splashing. Overfilling the bearing cavities with waterproof grease (enough to actively squish out) and adding some oiled pipe-cleaners (later, external o-rings stacked on the cones against the dust caps) as an additional barrier just about trebled the time between service intervals. I'm a big believer in bearing shielding (using a variety of means, from simple grease-in shear to mechanical labyrinth pathways to non-contact and contact neoprene shields with and without air gaps on cartridge and loose bearings). Combined methods seem to work better than single solutions. On loose-bearing hubs like my Sherpa's rear Shimano Deore, I especially like the extra shielding afforded by the cone-affixed, overlapping dust cap and have packed all the interstices generously with Phil Wood grease. The overlapping design retains grease and ensures it is nearly always in shear. I believe anytime a shield/seal can fit closely to a bearing, direct entry of water and other contaminants like dust will be minimized. Add grease, and there's another barrier between the bearings and the outside world; all good, but better if it can leak outward a bit.
Here are some additional thoughts...
I used to repair cars for money, and it quickly became evident controlled leakage was a hallmark of good all-weather seal design. So long as lubrication could seep out in a controlled manner, water and dust intrusion were held at bay. Transmission and differential seals come to mind; so do ball-joints fitted with zerk grease fittings, which allowed grease to be replenished and contaminants purged. Wilderness Trail Designs' GreaseGuard bicycle components used this concept and it worked well if one purged all wet grease after each submersion. I found well-maintained zerk-fitted automotive ball joints with large clearances did well because fresh grease displaced water and dirt, relubing the bearings. More modern replacements used tighter clearances and improved seals that better prevented the entry of water. When they finally died, they usually also exploded in a pool or soupy, rusty water and emulsified grease when knocked out for replacement. The seals had been compromised, accelerating wear through lubrication loss and dirt entry. I believe this is one reason why Rohloff hubs seem to do so well when exposed to a great deal of water. They are lubed with thinner oil (that can more readily flow and seep) instead of thicker grease; a hallmark of their operation is "oil-misting" around their seals -- controlled leakage.
Similarly, SON dynohubs use conrad-type cartridge bearings with metal-backed neoprene shields, and a pressure-equalization scheme to keep moisture from being drawn through the seals when moving the hub rapidly from one temperature gradient to another. There's a lot of air-filled volume in the large hub shell, and the tiny hole in the center of the axle provides a direct path for equalization rather than using the hub shields/seals to "breathe". Peter White has a nice piece on this here: http://www.peterwhitecycles.com/schmidt.asp
Scroll down to the section labeled "Reliability".
Of course, Jobst Brandt has spoken at length on the topic, including this exchange and discourse from Usenet's RBT some 8 years ago wrt sealed cartridge bearings:
These are not sealed bearings in the sense of water proof but rather
instrument bearings designed to prevent airflow through the grease
that acts as a dust trap. These bearings were not designed to be
exposed to water.
There is a design rule in seal design that states that:
1. The seal that doesn't leak, leaks.
2. No two liquids can be separated by one seal lip.
The first is based on the lubrication of the seal lip that works only
if it is lubricated. Therefore, some of the contained liquid must weep
under the lip to furnish lubrication. If it does not, the seal lip
will burn until it no longer makes contact... leaks.
The second is apparent from the first and requires a separate seal for
each liquid because they both must weep under their seal and if there
is only one seal, they will mix.
Jobst had a lot more to say on the topic, and his thoughts are contained in the Usenet RBT archives and in a summary version on Sheldon Brown's website: http://sheldonbrown.com/brandt/sealed-bearings.html
More here: http://sheldonbrown.com/brandt/index.html
There can be another problem that arises from bike submersion, and that is frame-filling. Most frames have small vent holes in the stays and forks to allow hot gasses to escape during construction. These same holes can allow water to enter and migrate to the bottom bracket bearings or to cause rust in the frame tubes. I have usually plastered mine shut with some kneaded beeswax to eliminate the problem. My Sherpa was cleverly constructed so the main frame junctions vent internally. The fork is vented at the top-inside, where the fork blades meet the crown. Getting water inside where it could not escape would be Bad. I transported a bumper-rack mounted bicycle the length of Interstate-5 during the eruption of Mt. St. Helens, and the frame arrived home filled with corrosive volcanic ash. Most entered through the frame and fork vent holes, and it was a terrible chore to remove. Water can do the same.
Myles, I have no doubt you've done well in your water crossings, and I honor and respect your experience; it has worked out well for you and sometimes not so well for others, and it has caused me to wonder why. It may in part be due to your bicycle and care in making sure the bearings are well-greased. Thorn's penchant for minimizing water entry to the frame coupled with the oil-filled Rohloff at the rear and a well-greased, nicely shielded hub at the front (XT) might be the reason for success. I really believe well-greased and oiled seals do wonders toward keeping bearings drier and cleaner. I think the amount of time submerged may be a factor, as well as the presence or absence of a swift current. It may also have something to do with maintenance schedules, as you noted here: http://www.thorncycles.co.uk/forums/index.php?topic=3324.msg14969#msg14969
All the best (and hoping to see more of your photos!),