There are a few places that tick most of your boxes, but unfortunately they will also tick your negative boxes as well.
The places that are most easily accessible in Thailand that have wonderful beaches and reefs you can access off the beach will also be over-run with tourists at that time of year.
The queiter locations will only be quiet because they will be owned by a larger hotel group that will welcome package tourists and children.
You may have to make some concessions to your want or not want lists to find a place at that time of year.
There are some great reefs at Koh Tao but the better beaches will be full of people who dive by day and party by night.
The southern beaches of Phuket are beautiful and you can even find some that are relatively quiet but you will have to take at least a 45 minute boat trip to Coral island to find the nearest good snorkelling location and it will be full of daytrippers from all the other beaches.
Khao Lak might suit but the snorkelling will have to be day trip based,but the nearby Simillian Islands are the real deal. This is all marine national park so no accomadation. You can do some 2-3 day trips where you stay on board the boat but this would be more diving inclined. You are likely to find some nice boutique accomadation At Khao Lak but most of the seafront stuff will be large hotel chains.
Then there is Phi-Phi. The northern part of the Island where the Holiday Inn is, is beautiful and quiet, excellent snorkelling but I would think there would be quite a few families to share any of these resorts with.
The smaller idyllic Islands will be hard to get to and you may find any acommadation there to be rustic at best. I havn't been to or heard much about Koh Jum. There is no good snorkelling on Samui and Koh Lanta is a bit of a family Island.

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