What method are you using to get rid of yhe water? My own underwater tunnels 3 wide 4 high i lay down yhe floor then use gravel down the middle with ladders on both sides leaves a breathable walkway then put in glas walls and ceiling then a simple matter yo remove gravel

It looks like he is using full sand unless thats his endcap @sknup

I used same gravel and ladders to remove all he water around a sea temple