What is two points of the same phase?
It need not be crest to crest , it can be trough to trough or any other 2 points to the same phase. Having same phase means the points have the same displacement and velocity. There are many points have the same phase , so choose one which is closest i.e less than half a cycle away. If the phase angle is 90 degrees the other point is 270 degrees. If phase angle is 30 deg that the other point is 330 deg . Normally a smaller angle will be oonsidered. Phase angle can occur within one wave or between two waves. If the phase angle is bwt two waves then the is timing difference. If it is within a wave that it is a distance apart. Select 2 location along the beach one may get the wave peak first and then the other . A person standing on the beach may experince a peak when a boat passes by and then again when the next boat passes. Hence there is differenc in timing.
