Anyone know how to make an air bar for an underwater game? Thanks in advance!

basically, I'm making an underwater game where you need to go underwater find treasuer and other stuff. but I don't want them to stay under all the time. I want a bar that tells them how much air they have left and when it hits 0 they lose health you gain air when you are out of the water and loses it when your underwater

-- I think the code might be something like editing the bar size
bar_one = script.parent.WaitForChid("BarOne")
if  == .............. do
bar_one  --size decrease
else
bar_one --size increase

end

I'm not sure if that's on the right lines using a terrain water to make it look realistic