First of all....stoked you are coming to the island! There are many options for great operators here and I'm sure you'll have a great experience. I've worked for just about everyone on the island in the last 19 years at one point or another and I am lucky to know most of the DM's here on the island. While I am with Bubbles Below Scuba currently, I'm happy to help with any questions you may have about any of the operators and/or put you in touch with anyone you may be curious about or have questions for.
April is generally the kickoff time for Niihau and Lehua diving. More accurately, the kickoff is simply as soon as the last of the huge north swells leave for the season. Bubbles Below does go year round when conditions permit. The other operations that cross to Niihau include Explore Kauai Scuba and Seasport Divers and Fathom Five. Conditions during the time of your visit may very well dictate which operator you are best suited to cross with. There are several different options for boats as well as passengers that they take. The closest boat leaves from Kekaha, while the furthest trips leave from Poipu. When it is rough, it can be nice to shorten a "sometimes" memorable if not "brutal for some" crossing. Big winds in particular do not make for optimal crossings. If you have excellent sea legs and have done rough water crossings/trips before - this may not be as much of an issue. One of the reasons Niihau is so amazing is after all - you can't always get there. Minimal diver impact and presence does play into the wow factor of Niihau. As far as the diving in April, you do have a chance at seeing Humpbacks over there during that time which is simply amazing. It is the end of the season - but late season Mom's are often around still with their calf for example. The monk seal activity is sometimes limited in April as they often haven't returned to Lehua rock and the northern parts of Niihau just yet. That's not to say you won't see them, but it's not as likely as mid summer.
Shore diving is actually pretty limited on Kauai. The southern end of the island (Poipu/Koloa) has an awesome - awesome - awesome critter dive! It's shallow and relatively protected and boasts some rare critters and countless turtles! Vis is generally good and access is easy. The name of the site is Koloa Landing. While there is a dive on the north shore called tunnels, it is primarily a topography dive with lava tubes. If you love tubes and topography it is pretty cool. At the same time - you should be aware that vis' is typically quite low compared to any other dive you will do on Kauai. Many people love it, but to be fair and open - I hear a lot of divers discouraged about how poor vis can be. The backdrop for the dive is breathtakingly beautiful and the beach is quite nice. Snorkeling can be quite nice as well.
Depending on where you choose to dive here are a couple of the signature dives for Kauai....
1. Hale O' Honu (House of the Turtles) - Simply the most turtle intensive dive on the island! It's in the 15-50ft range and typically has whitetip reef sharks and at least 1 if not more Frogfish. You can count of Endemic Bandit Angels and also beautiful Hawaiian Lionfish (the good kind). Additionally, it's your best chance on the island to see the critically endangered Hawksbill Turtle!
2. Sheraton Caverns - Lava Tubes and Turtles. Probably the best known site on the island. While it can get quite crowded at times, it is a really terrific dive. Two adjoining lava tubes with large skylights allowing for plenty of ambient light inside. Lots of turtles and inverts on this site. It is shallow as well, with the max being around 65ft if you head outside to see octo's hiding in the rubble. Large Puffers are also common here.
There are certainly numerous other great critters on these sites as well. The shallow nature sure gives you plenty of bang for the buck as you will get long bottom times.
Feel free to send me a personal message if you like with any specific questions. I'm sure you'll get other helpful responses from some of the other awesome dive professionals here as well!
Good luck planning and have a wonderful weekend
A
