I have 2 questions on this small GUI script?

If a player is touching the water part, a blue bar GUI will appear on their screen, showing their air. If they are in for too long, it will start to deal damage until they get out of the water. However, it doesn't show the air bar decreasing, and it also doesn't stop dealing damage if they get out.

local air = script.Parent
local hitbox = game.Workspace.WaterHitbox
local airBarSize = air.CurrentAir.Size.X.Offset
air.Enabled = false

hitbox.Touched:connect(function(part) 
	local humanoid = part.Parent:FindFirstChild("Humanoid") 
	if (humanoid ~= nil) then	--if a humanoid exists
		air.Enabled = true --show air bar
		repeat
			wait (0.01)
			airBarSize = airBarSize - 2
		until airBarSize == 0
		repeat
			humanoid.Health = humanoid.Health - 1
			wait()
		until not airBarSize == 0
	end 
end)

hitbox.TouchEnded:connect(function(part)
	local humanoid = part.Parent:FindFirstChild("Humanoid")
	if (humanoid ~= nil) then
		airBarSize = 544
		air.Enabled = false
	end
end)

This is due to line 3. You’re setting airBarSize to the value of X.Offset at the time you assigned it the variable. For example, if the current value of the Offset was -25, local airBarSize = air.CurrentAir.Size.X.Offset would be the same as local airBarSize = -25. Another thing to add on is that your script will error if you try to assign X, Y, XOffset, or YOffset from a script. These cannot be written from scripts, but manually in the Explorer. You’ll have to use UDim2.new() to set the size.

What I would do for line 3 is simply set it to the object:

local airBar = air.CurrentAir

But you can out this question I asked to get a better understanding of this common mistake:

https://scriptinghelpers.org/questions/61116/when-do-you-store-a-property-as-a-variable