It may look like this, especially at a neap tide.
But at a spring high tide it may look like this
Yes the saltmarsh is completely submerged.
If saltmarsh is submerged for days, weeks or months it will eventually die.
But when it is just submerged at spring high tides the mud (or sediment) in the sea water
gets deposited on the saltmarsh and causes it to rise very slightly.
In the UK it is sometimes said that rising sea levels kill saltmarsh, but this is only true if sea levels
are rising faster than the saltmarsh can rise from sedimentation.