the tide was at the ebb
This should be, ' on the ebb'. With the use of 'at', which implies a 'point' in the process of the tide going out, the author seems to be meaning that the tide had ebbed to some low point, if not 'was out'.
Can I say "the tide was low"? NO
This is the opposite of a 'high tide' or "king tide', and refers to how 'high' the tide is when it 'comes in' - it was not a high tide, it was 'a low tide'.
Try - 'the tide was out'.
When used figuratively, as when referring to feelings etc, then
it refers to 'in a bad or weak state':
Consumer confidence is currently at a low ebb.
I was recently divorced and feeling at a very low ebb.
...and 'at' is the appropriate preposition.