A few tips for you to enjoy next Hawaii trip better

1) take ginger pills daily starting a few days before your boat trip. Very effective for motion sickness.
Or you can get preion, even over the counter motion sickness drugs such as antihistamines or get ear patches.

2) There are plenty of places in Kauai (and Hawaii as a whole) to have excellent snorkeling - just not in the middle of the ocean!
For good snorkeling, you go to SHALLOW, rocky bottom beaches where there are plenty of rocks for underwater plant growth - so the fishes have foods! Sandy bottom beach wont have many fishes as there are no food. Deep ocean has plenty of fishes but they are too down below that you, at the surface, would not be able to see.

3) We have excellent snorkelling at Kauai - LG literally stepped on a turtle at Poipu beach on his swimming back to shore after finishing snorkeling. At the north shore of the island, (where the trail head to Na Pali Coast starts) Hanalei beach has good snorkeling also. However, the north shore has more rain than the south shore (Poipu).

4) Helicopter ride at Kauai is the ONLY way to really see Waimea Canyon AND the dramatic showing of Na Pali. At the end of the ride, you fly thru the narrow opening of the canyon and suddenly the Na Pali Coast is right at your face.
However, there is a risk involved in doing this type of excursion. Though Hawaii is better than Alaska, in that, the weather is more predictable.