I've had a few females with long crests.
I agree that hissing is just the way the boys talk. Its not like goose or cat hissing. Head bobbing, sky beak clapping, and tail wagging are good also.
To me aggressive behavior would be when they lower their heads and stretch their necks out in front of them parallel to the ground. Raising their wings slightly away from their body like a cape while running toward you. Or leaping at you feet first.
Both hens and drakes do make a type of short warning huff, that makes the soft skin under their beak inflates a little.
From what I've noticed, even the sweetest most attached boys go through a major personality change when breeding hormones kick in. They are very nippy grabby at first. You can train them not to treat you like a female duck that belongs to them, but it takes time. They do change a lot.