November is too late for Niihau unforunately. Unless you want to dive in 20-30 foot swells. Most operators do their last trips in the end of September. The majority of the diving is done on the south shore of the island in the winter months. There are 3 operators, Seasport Divers, Fathom Five, and Bubbles Below that are worth diving with. All do a great job. Seasport Divers has a 48 foot pro MAX 16 divers and a 32 foot custom powercat MAX 10 divers - the groups are never larger than 7, and usually smaller. The other operators have small 6 PAX boats. Due to the wind, the seas on Kauai get bumpy, I would want the larger boats so you have a way to get out of the weather and move around during your surface interval. Seasport would be my choice.
Kelp does rule - being from Laguna beach, that's the one thing I miss - its just too d--n cold