Tide

According to the dictionary:

The alternate rising and falling of the waters of the ocean, and of bays, rivers. The tide ebbs and flows twice in each lunar day, or the space of a little more than twenty-four hours. It is occasioned by the attraction of the sun and moon (the influence of the latter being three times that of the former), acting unequally on the waters in different parts of the earth, thus disturbing their equilibrium. A high tide upon one side of the earth is accompanied by a high tide upon the opposite side. Hence, when the sun and moon are in conjunction or opposition, as at new moon and full moon, their action is such as to produce a greater than the usual tide, called the spring tide. When the moon is in the first or third quarter, the sun's attraction in part counteracts the effect of the moon's attraction, thus producing a smaller tide than usual, called the neap tide.

Tides affect:

• where the edge of the water is - where you can get in and out
  • (also the depth of water on this sandbar where I just pitched my tent/left my kayak 6 hours from now)
• depth of underwater obstructions
• current through passes or channels
• height of waves when current opposes wind
• surf condition (depth of obstructions to waves)