wvmarle:
The moment you have your line fully wheeled down, you have a 1000 ft line with a 1 kg weight on it. Even at your 400 ft (well over 100 meters) limit that's indeed bound to start swinging around, and your drone may not be able to compensate for that. That's one massive pendulum and there's always wind, if only from the drone's rotors, to start a swinging movement.As you mention it's to be lowered under water - so you can mitigate this problem by having it winched all the way up when starting off, then lowering your drone as low as possible above the water and start unwinding. The water should stop it from swinging - but you will have a serious problem if there's any current in the water you lower your load in, as the pull of the current will easily overpower your drone.
Sorry, I should have clarified earlier. As you said, I won't actually be flying around with the payload extended to any significant length. This is actually one of the reasons I'm not fond of the static line length designs I've seen. I would be hovering the drone 5-10m above the water and lowering/raising the payload from there.
The bodies of water I'll be using this in shouldn't have much of a current, but that is a concern of mine. A larger concern I have is the payload getting caught on something under the water. Perhaps I should be thinking about some type of emergency release mechanism...