The 5D Mark III is weather-sealed against dust and moisture, making it splash-resistant, but it is not waterproof and cannot be submerged.
No, the maximum video resolution for the Canon 5D Mark III is Full HD (1920 x 1080) at 24, 25, or 30 fps.
No, the 5D Mark III is a full-frame camera and only accepts Canon EF mount lenses. EF-S lenses designed for crop sensors will not fit.
The LP-E6 battery provides approximately 950 shots per charge according to CIPA standards, though this decreases when using Live View or recording video.