The primary benefit of going to Kauai during high tide season is the ocean. It is stunning -- the power and force with which it bursts onto the shore is remarkable, especially when combined with the steadfastness of the mountains. We went to Queen's Bath and sat on top of the rocks for about an hour, just watching the ocean churn. It was spectacular.
However, it precluded us from doing any snorkeling or swimming in the ocean (at least on the north side of the island, we never tried to swim on the southern side, e.g. Poipu), which was disappointing. I had read that Tunnels Beach yielded some of the best snorkeling in the world, so it was a letdown to not be able to see that. However, instead of spending the day at the beach, we amused ourselves in other ways, such as hiking or going into town. It was really a perfect time to go hiking since the weather was about in the mid- to upper-70's the entire time we were there. Thankfully, we did get to snorkel in Maui, and tried to swim a little bit, but the tide was very strong, although less violent than it was in Kauai.
I was also disappointed to not see any whales in Kauai the entire time we were there. It is possible that our untrained eyes skipped over the whales that were out there. Happily, we saw many whales in Maui's waters, which sees more "whale traffic" than does Kauai, which is the most remote of any of the islands.
All in all, I loved getting away from the midwest winter! The next time we go to Kauai (I do hope there will be a next time!) I would like to go during the low tide season, to see what it's like and to be able to swim in those beautiful clear waters.