Nixe? Like water nymphs? Is this a mod you're using, or have you simply misspelled "neck"? Necks don't do a lot of damage, so I'm wondering if it isn't the former. Creatures don't do damage to things unless you've gotten close enough for them to spawn in and become "active", and you'd have to be ignoring a neck for it to have enough time to ruin a boat. If it is just necks, you might consider leaving some wolves nearby. They'll handle necks no problem.