The smallest high tides occur when the moon phase is

 when the moon is dark

when it's full moon it's the highest tides. The moon attracts the ocean..

Geothermic gradient is the rate of increasing temperature with respect to increasing the depth in the earths interior. away from tech tonic plates boundaries, it is about 25-40 c/km (72-87 degrees f/mi) of depth near the surface in most of the world.