In November, I suggest avoiding Thailand's east (gulf) coast -- Koh Samui, Koh Phangan, Koh Tao, etc. -- as the weather can be stormy and the seas can get quite rough, and the U/W visibility can drop to a couple meters. (Pattaya and some other northern parts of the gulf might not be so affected, but I wouldn't recommend Pattaya as a honeymoon destination ...). The weather in the gulf can be perfect in November, but often it isn't. It also can be a bit spotty on the west coast in early November, but by later in the month as the monsoon ends, it's usually OK.
So -- on the west coast, if you want a low-key romantic environment with good beaches, few neon lights, and great diving, I can vouch for both Koh Lanta and Khao Lak. Koh Lanta is better if you want to spend all your nights on land, and Khao Lak is better if you want to spend a few days on a liveaboard dive boat in order to get to some more remote divesites.
I haven't dived Koh Kanta in a few years, but assuming that its soft corals and other marine life have recovered from the 2006 red tide, I'd say that Lanta is probably better-suited to your interests than is Khao Lak.
On Lanta, you'd be spending a bit of time on the dive boat to get to and from the dive sites, but would be back on the island in the afternoon to enjoy the beaches and restaurants and then to sleep in your own hotel room bed when it's bedtime. In contrast, Khao Lak is really best as a place to spend a day or two relaxing before or after doing a liveaboard to the Similan and Surin and other islands in Thailand's northern part of the Andaman Sea (although from Khao Lak there are some dives you can do as day-trips, or do land-based activities). As it sounds like you don't want to be on a boat for a few days in a row, I'd recommend Koh Lanta over Khao Lak.
I think the diving you can do from Koh Lanta is as good as what you get in the Similans, etc. (or, at least it was as of a couple years ago when I was last there). Apart from manta rays, I never saw anything in the northern divesites (Similans and Surins and Richelieu Rock) which I hadn't seen in the southern ones (Koh Lanta, Koh Phi Phi, etc.). If mantas are very high on your list, take note that they are sometimes seen on some southern sites, but you are more likely to see them around Koh Bon, which is a northern dive site best accessed from Khao Lak or Phuket on a liveaboard (west coast divers will correct me, I'm sure, if I'm wrong -- but I think Koh Bon is either part of the Surin Islands or is between the Surins and the Similans). Of course the far-north Richelieu Rock is a world-famous divesite for good reason -- it's truly phenomenal -- but you can only dive it from a liveaboard, and I've seen all of the same marine life at Lanta-dived sites like Hin Daeng, Koh Haa, etc.
For more nightlife, a wider variety of restaurants, and other tourist attractions, you could of course opt for Phuket. There are some nice day-trip dives done from Phuket which aren't dived from Khao Lak (the King Cruiser wreck, Shark Point, and others). However, unless you're staying at Le Meridien or another many-star resort with its own private beach, then you'll have to deal with crowds and neon and all else that comes with overdeveloped tourist destinations. Some folks love it, and some don't.
I can't really speak from experience about Koh Phi Phi, other than to note that the dive operators there go to the some of the same divesites you'd dive from Phuket, and some of the same sites you'd dive from Lanta. I don't know the island, but from what I gather, Phi Phi's a beautiful small island with no motor vehicle traffic, an entirely tourism-based economy, and big crowds and high prices. Like Phuket, some love it and some don't.
Ask again if you have follow-up questions, and if not then please post a trip report after your honeymoon.
Cheers,
M
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