Diving Similans October illegal or no?

Same dates, in fact by heart I think the accommodation on Surin is not available till the 15th of November.

We have been camping there since the first week. That does not mean the bungalows are open yet...but we avoid booking them and use the other beach instead.

As for date limitaitons - as Steven said - the Surin Islands are closed until November 1st.

The snorkeling is better on the Surin Islands...with one caveat. If you go for just a daytrip, you will end up on a crowded speedboat with dozens of other people, 45 minute "snorkels" and guides who are too busy to show any marine life. This is the same kind of trip on either the Similans or Surins...so they are both kind of lame. If you spend a few days, use a longtail, get in a small group - then the Surin's are very much worth the effort.

I know they have 2-3 day snorkel liveaboards to the Similans but I imagine that bioles down to 1.5 days out at 2 or 3 of the islands and maybe 4-5 snorkels of 60-90 minutes each. I did not see a 2 day liveaboard to the Surins. We are very spoiled New Yorkers and we expect the boats to have air conditioned cabins which some Similan liveaboards boats are equipped with. If We can work it without more than one connection we are first going to the Raja Ampat Islands on a snorkel livaboard of 12 days and then from jakarta would like to fly directly to Phuket (that may not be possible without going through BK) and spending 7 days in that area. because of the fare structure we will have to fly back to to jakarta and then on to NYC. At age 70 that's not a "piece of cake"..

There are Jakarta to Phuket flights.

Most liveaboards in Thailand are scuba-dedicated. They will take non-divers/snorkelers, but very few pay much attention to them. There are two multi-day snorkel boats in the Similans/Surins that I know of, and one dive boat that has a mission of taking good care of its snorkelers. Choose carefully.