The way I avoid the clew from going underwater is to have the clew towards slightly towards the wave (towards the wind in side-on conditions).
That way the clew is rarely fully underwater (that would mean over 2m depth needed anyway!).
If it is, then we need to choose which way will be the easier to get the sail to rotate and lean it against the waves/current.
I grab the mast and pull hard towards the waves, it does take a couple of seconds. Once it's at a slight angle then our legs can go below and our "helicopter"-style becomes more effective, from there it's only a few seconds until it's fully out.
One tip, I only start lifting the sail up only once the boom clip is out of the water, this ensures the clew releases nicely once the wind gets under. For max leverage I try to set the mast really close to the wind maybe 20 degrees off.
If the mast is completely down (now needing over 4m depth!!!), then I grab the (upside down) board and pull on the rail, the mast will go horizontal and the clew quite down. So I use the waves as if they were wind to move the sail around.
You can see how I lean on the mast and swim with the sail under water here, although it's not fully dedicated to waterstarting: