sea depth chart measurements

New to reading sea depths,for instance what does -6.7 mean?

The reason I am asking is so I can find holes and sandbanks to fish when I launch out of Rhyl harbour.
Plus the channel in and out.
Any advice will be appreciated.
"V"

I've done a bit research,the measurements with a minus sign are where the tide goes below the datum.
To find the depths at high,you add the height of the tide to the map height.
So,if for instance,the map says 6metres,you add the tide height to it.

i.e. 6metres+ a tide height of 3 metres= a depth of 9metres.( about 29ft) which is the depth you would be fishing at.