"Witches Aboard" follows the Lancre coven across the Disc to the Big Easy, well Terry Pratchett's version of the Bayou, as they attempt to save the day by not allowing the servant girl to marry the Prince. The happily-ever-after and fairy godmother tropes do not survive either Pratchett or Granny Weatherwax.
I will be honest, the previous Witches' book (Wyrd Sisters) wasn't my favorite Disc book and so I had reservations when beginning. After what I felt was a stumbling start with numerous "mirror magic" asides that didn't help explain anything, the main story of the Lancre coven's cross-continental trip to the city of Genua took over and really grabbed ahold of me. The interactions of Granny, Nanny Ogg, and Magrat amongst themselves and with "foreigners" is hilarious especially when they come into contact with the local witch in Genua, Mrs. Gogol (the voodoo priestess). Once in Genua, the coven members realize that someone is making fairytales become real life by any means necessary resulting in a whitewash, forced-into-happiness city instead of the generally jovial one that most residents once remember. The revelation that a relative of Granny's and the duality of fairy godmothers just adds more spicy to this story. However, the best thing throughout is the tomcat Greebo especially when he gets into human form.
After the inconsistent start with the weird "mirror magic" explanations that really didn't help anything, the only other complaint was again Magrat's character because not only did she remain flat like in Wyrd Sisters but she was almost reduced to "minor" status by the end of the book. Given that she is one of the titular characters, it's a bit weird seeing being less relevant than a cat.
Besides those two gripes, "Witches Abroad" is a improvement in my view over "Wyrd Sisters" in Pratchett's Discworld series. The vast majority of the Discworld books I've read, I've enjoyed so to anyone looking to get into Discworld please don't considering my not-so-liking of the Witches books be a stain on the other Disc books.