The mean between high and low tide is used when applying for a building permit to check if the building is sufficiently away from the waterline. Every area has zoning laws that regulate that. Here in the gulf in our place you have to be 20 meters away for building up to 6m (single storey) or 50 meters for building up to 12 meters (double), 200 meters away you can build higher. There are more exceptions to this, structures under 1.2m height, like a pool or terrace, if not too big don't need a building permit, as they are ground level. That means also the distance to the mean tideline is irrelevant as long as you build it on your property.
However, this waterline changes dramatically between the seasons, depending on where the wind is blowing from. That can easily make a 20 meter difference between high and low season. Usually in November/December the wind is coming from the north/west and in rainy season from the east/south.
Just read one of the many coastline erosion studies from thailand like https://www.mdpi.com/2077-1312/11/5/969
Now if you got your building permit 10/20 years ago, everything might have been fine and totally in line with the zoning laws. That might not be the case anymore as erosion moved the beach inwards into the countryside, where you have part of your land. What are you going to do except the usual anti erosion measures you can do? Tear down your Hotel or Condo because it suddenly looks too close? All those houses at the chao praya river in Bangkok that were allowed to build like that in the past because zoning laws where more relaxed, tear them down?
Whats there to back up? Beaches move inward all the time, beach erosion is a giant issue in Thailand. Pattaya spends hundreds of millions every where to fill up the sand with questionable results, what might have been far away from the beach 20 years ago might not look like it is today.
You can check on landsmap: https://landsmaps.dol.go.th/ it gives you the chanote outlines of most places in thailand.
Just look at this random chanote in Samui:
Half of what people call the "beach" is on the chanote, doesn't mean the owner can't build an ugly concrete terrace with even even uglier pool there till his land border.
Would prolly get the same reaction here... doesn't make it illegal tho. When it's high tide it could very well be that the ocean runs into that guys property, and if not now then it 10 years.
Zoning laws are pretty clear in that area, no building (buildings - so everything over 1.2m height above ground) 10 meters from the beach. Not bigger than 75 sqm per building, you can building multiple dwellings that have a distance of at least 4 meters from wall-to-wall to still build a "bigger" house.