The 2nd, 3rd, and 4th hour is when most of the water is moving (2/12 + 3/12 + 3/12 = 8/12 = 2/3rds of the tidal change). The two hours surrounding the high/low tide is what is referred to as the 'slack tide', when not much water is moving. (1/12 + 1/12). Weather also impacts the tides.