You're thinking one of two things right now:

This is the best idea for a product in the world.
Who the hell would buy something so stupid?

Assuming you're in the category #1 of people, here are some of the details (as listed by Amazon).

"Great for workouts away from home." (This is the main reason I would want to own them. You can take these anywhere you go and still do your body business? Amazing.)
"The perfect travel companion." (The PERFECT travel companion? God, I hope not. Bit sad of a thought. I still want them, but nobody should yearn for them.)
"Each holds up to eight pounds when full." (Eight pounds is ridiculous and amazing.)
"Guaranteed not to leak." (Sweet.)
"Aquabells empty in seconds." (Love it.)

I was on the subway and thought of the idea of water fillable hand free weights (those exist too, but the ones I found looked too meaty, these ones seem better), and of course the product already existed when I did a Google search for them. Such a smart idea. You can get them on Amazon for $28.50.