Indexing Sentinel 2 image collection using several conditions

Question

I want to mask water bodies with two different conditions and then add the two indexed layers together to create one layer with different values of 0 (no water), 1 (water: based on index 1), 2 (water: based on index2) and 3 (water: based on both index 1 and 2). here is the code, I don't get any error but the final water band has pixel values of 3 for all the pixels. I can't figure out where the problem is. What am I doing wrong?

s2 = s2.filterBounds(region)
     .filterDate('2021-01-16', '2021-01-18')
     .filter(ee.Filter.lt('CLOUDY_PIXEL_PERCENTAGE', 35));


function mNDWI(img){
    var img = ee.Image(img).divide(10000)
    var mndwi = img.normalizedDifference(['B3', 'B11']).rename('mNDWI')
                .copyProperties(img,['system:time_start'])
    return img.addBands(mndwi)
}

function Fisher(img){
    var img = ee.Image(img).divide(10000)
    var fisher = img.expression('1.7204 + 171 * Green + 3 * Red - 70 * NIR - 45 * SWIR1 - 71 * SWIR2',
                          {'Green':img.select('B3'),'Red':img.select('B4'), 
                           'NIR':img.select('B8'),'SWIR1':img.select('B11'),
                           'SWIR2':img.select('B12')}).rename('Fisher').copyProperties(img, ['system:time_start'])
    return img.addBands(fisher) 
}


var ic_in= s2.map(mNDWI).map(Fisher)

function water_thresholding(img){
    var img = ee.Image(img)
    var rvrs = img.select('mNDWI').gt(-0.3).rename('rivers')
                                          
    var fi_index = img.select('Fisher').gt(0.63)
    var mndwi_index = img.select('mNDWI').gt(0)
    var otrs = (fi_index.or(mndwi_index)).remap([1, 0], [2, 0], null).rename('others')
    
    var water = (rvrs.add(otrs)).rename('Water')       
    return img.addBands(water)
}

var s2_ic_msk = ic_in.map(water_thresholding)

print(s2_ic_msk.first())

Map.addLayer(s2_ic_msk.select('Water'), {}, 'Water' )

Answer 1

Your issue is produced because you are dividing by 100,000,000 instead 10,000; as it was well pointed out in comments. One of different ways for avoiding that is combining all functions in only one as follows.

var s2 = ee.ImageCollection("COPERNICUS/S2_SR");
var region = ee.FeatureCollection("projects/ee-sshaeri63/assets/Gwydir");

s2 = s2.filterBounds(region)
     .filterDate('2021-01-16', '2021-01-18')
     .filter(ee.Filter.lt('CLOUDY_PIXEL_PERCENTAGE', 35));


var s2_ic_msk = s2.map(function (img){

  img = ee.Image(img).divide(10000);
  var mndwi = img.normalizedDifference(['B3', 'B11'])
    .rename('mNDWI')
    .copyProperties(img, ['system:time_start']);

  var fisher = img.expression('1.7204 + 171 * Green + 3 * Red - 70 * NIR - 45 * SWIR1 - 71 * SWIR2',
                           {'Green':img.select('B3'),'Red':img.select('B4'), 
                            'NIR':img.select('B8'),'SWIR1':img.select('B11'),
                            'SWIR2':img.select('B12')})
                            .rename('Fisher')
                            .copyProperties(img, ['system:time_start']);
  
  var rvrs = ee.Image(mndwi).gt(-0.3).rename('rivers');
                     
  //var wtls = img.select('TCW').gt(-0.035).remap([1, 0], [2, 0], None).rename('wetlands')
                        
  var fi_index = ee.Image(fisher).gt(0.63);
  var mndwi_index = ee.Image(mndwi).gt(0);
  var otrs = (fi_index.or(mndwi_index)).remap([1, 0], [2, 0], null).rename('others');
    
  var water = (rvrs.add(otrs)).rename('Water');  
  
  return img.addBands([mndwi, fisher, water]);
  
});

var mean = s2_ic_msk.mean().clip(region);

var imageVisParam = {"opacity":1,
                     "bands":["Water"],
                     "palette":["ff8a23","ff2710","1814ff"]};

Map.addLayer(region);
Map.addLayer(mean.select('Water'), imageVisParam, 'Water' );
Map.centerObject(region, 9);

Result is the same with the previous three functions (corroborated by me) and, it can be observed in following picture.