hello,waterproof refers to the surface of the filter has a waterproof film, if water drops on the filter, the water will directly slide off without leaving water marks on the filter, then it is easy to clean.we do not recommend that you use in the water, the water will enter the CPL inside the seam. 

Built in. Only need sun or plug in charger ( it comes with one) to power up. 

Depends on what you mean by direct sunlight. If you mean the filter pointed directly at the sun, and you are worried about lens flare, well it wont stop that. But if you mean using it out in bright bright sunlight all around, then it works great. I'm a member of the Longview Film Club. We shoot indie short films, and use these for all of lenses and cameras. It's a must when shooting outside under almost any conditions. Makes it possible to get nice Depth of Field even on a sunny day. 

Yes and no. Using motion detection it doesn't work through a window. However.... using the timer inside it you can record through a window using it. How? Set the timer to record 4 minutes every 5 minutes for 1-5 hours or whatever time you need it to record for. This is what I do- just make sure you have a SD card with enough memory for the videos- I use a 32 GB SD card, I use it though a window as a security camera for my car....I live in an apartment so I can't hard wire anything and this works great for what it is. 

hello ,you can use ie take picture of the sun, but not for the solar eclipse