Rather than ocean currents, this is a simple chart or table that contains predictions of the times of the tides (high tide, low tide - or in Japanese満潮, 干潮) at a certain stretch of coastline on any given day.
Does this clear things up?
Regards,
Alan Siegrist
Carmel, CA, USA
I'm only writing to add that 潮 can sometimes be used allegorically to
mean chance or opportunity. Depending on the context, your character
might be referring to something like "checking which way the wind is
blowing" rather than the literal rise and fall of the tides.
John Stroman