It may have something to do with swimming speeds as well as the speed with which a barnacle larva can attach to whale or dolphin skin. I've never actually smelled one. However, live dolphins don't have much of an odor because they don't have skin glands. Dolphins have few predators to begin with. I would guess that whenever possible the dolphins will avoid predators rather than try to fight with them.
Dolphins play with lots of things, including seaweed.
Seaweed includes a variety of aquatic plants. However, I've never heard of a dolphin becoming entangled in it. I don't know when the first dolphin was discovered. Probably no one knows. Yes, dolphins sleep but apparently not the same way that humans do.