which camera is better canon 7d or nikon d5100

d5100 is cant survive rain directly w/o external housing - not sure of canon

None of the camera is "water proof".  Water proof means you basically go diving with it.  Canon 7D has weather sealed, which means it has good resistance against rains and other harsh conditions.

For beginner, Canon 7D may be overkill (like buying a Ferrari to drive 35mph), as you may not even need to use half of its features that make it so much more expensive than the D5100.  Technically, 7D has very fast shutter speed and fast autofocus, suitable to photo sports and other activities.  Its weather seal helps photographers to work outdoor.

But when it starts pouring rain (like in Asia), don't count on the weather seal to protect your expensive equipments fully.  And for under water photography, you need to buy a water proof kit.  And they can cost $500+ as well.  Since there is not a whole lot for photography under water, you may want to buy a cheaper camera specialized in underwater photography that it already comes with a water proof body.  A point and shoot camera with some optical zoom features would do. 

For video, personally I would go for the GoPro Hero video cam with its underwater kit, the entire thing may cost under $500.  GoPro makes awesome video camcorder that handles 1080P video.

So to answer your question, you have to ask yourself if you really need the super fast frame rate (how many pictures shot per second, like a professional sport journalist), do you really expect yourself to shoot under a monsoon, do you even expect your camera to shoot 1000+ shots per day?  This is where the 7D shines. Else, D5100 has an articulate screen and it is fun.

There is a big difference between the terms water-proof versus weather resistent / weather sealed.  Water-proof implies the camera can withstand water entry under certain amount of pressure, whereas, weather sealed means it can withstand cartain amount of water splashing or dripping on the camera without excessive driving force. Except for certain P&S models, none of the DSLRs can withstand being dunked in the water.  Even those water-proof P&S models can only survive to shallow depths.

D5100 is not weather sealed.  It's big brother D7000 and Canon's 7D are weather-sealed to comparable level, with 7D being slightly more so. 

Pentax K-5 is an excellent weather-sealed camera, especially pared with Pentax' own weather sealed lenses designated as WR lenses.  For Canon, only the expensive pro series Canon L lenses are mostly weather sealed whereas many newer Nikon lenses, even without being a pro caliber, are weather sealed to varying degrees.   While Canon and Nikon have more lense choices, Pentax does have a good number of WR lenses that will cover most all shooting needs.

If your camera body is weather sealed but if your lens is not, then it doesn't really help you when taking photos in wet environment.  As for IQ, Pentax k-5 has the same SONY fabbed sensor as D5100/D7000.

D7000 is rain proof, and is a lot better than the 7D in thermes of image quality, ISO and durability. Nikon D5100 is the little brother of the D7000, not as strong and fast.

Go for the D7000, but no dslr will let you take pictures underwater unless you have an underwater housing for that model (more then 1000 dollars for the cheapest).

7D is weather resistant but not sure if it's "waterproof". It can take some rainy weather as long as your lenses are weather resistant, but neither cameras can go under water without special housing for underwater shooting. As long as I know D5100 is not weather resistant camera, but if offers great in-camera creative mode for beginners. 7D is not really for a DSLR beginner, but if you can afford it right from the beginning, I would say why not!