cotidal

from The American Heritage® Dictionary of the English Language, 5th Edition.

• adjective Indicating coincidence of high tides or low tides.
• adjective Of or relating to a line that passes through each location on a coastal map where tides occur at the same time of day.

from The Century Dictionary.

• Marking an equality of tides.

from the GNU version of the Collaborative International Dictionary of English.

• adjective Marking an equality in the tides; having high tide at the same time.
• adjective (Phys. Geog.) lines on a map passing through places that have high tide at the same time.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective Describing the locations (and of related lines on a chart) linking places where tides occur at the same time of day