Pompei is simply undescribable, simply amazing. Forget any bad reviews, I have no idea what people could have been expecting and why they were disappointed. At any rate, given that the place is relatively far and huge (you'd need at least 3-4 hours to get an idea), I'd suggest Ercolano instead. Closer, smaller and in some respect even more impressive for a first-timer. That will give you enough time (and will not be so tiring) to allow you a walk through the city of Napoli (with or without the National Museum) and time to enjoy a good pizza and possibly a visit to the cappella Sansevero (cristo velato!!!). Then back to your boat and enjoy the sunset...

Goethe wrote see Napoli and then die, he didn't write see Capri or the costiera amalfitana and then die.