Most people think of the date April 15 as being painful in regard to being the day when you need to pay your taxes, or at least have them prepared by. For people on Kauai April 15th brings another kind of pain, because it was on this date in 2018 that an extreme flood, 50″ of rain in 24 hours, swept into the islands north shore and collapsed cliffs, ridges, bridges, excavated valleys, deposited copious quantities of run-off where it had never been seen before and pretty much just devastated all access to the NaPali Coastline from land or hiking trails. It has been very treacherous ever since and when it may reopen is not yet clear. Even when it does open, this type of radical weather event in the NaPali area is not unusual (though not this serious in recent memory) and this area can be closed down at any time as a result of flooding, landslide, wind or wave conditions.
Given this, the only real viable way to visit the NaPali Coast is by air or by boat. These trips are absolutely spectacular and you should definitely plan to take one. But even as I say this, the trips that visit NaPali by sea are limited, in general anyway, because of the extreme surf on this coastline in the winter months. The waves make it difficult and dangerous to visit on many days during that time (some days it will be doable and very beautiful as well but it is hard to count on these days being available during the specific vacation days you may be planning to visit).
If you are visiting Kauai during the summer months, say between May and October, you should try to make reservations for the many snorkel excursions, sunset sails, dinner cruises, kayak tours and other ocean-going tours that are offered during this period. Of course you will need to make these reservations well in advance so you can be assured to have space. So many people unfortunately wait until they arrive on island to make reservations an it is simply too late.
Your other option for visiting the Na Pali Coastline is of course by air on a helicopter or on a sightseeing airplane tour. These tours are available throughout the whole year and are a spectacular way to visit this amazing area. During the winter months on the days when the surf is pumping, it is particularly interesting to see this spectacle from the air. These tours are fantastic.