The problem with tides is that they are greatly affected by local bathymetry and are not usually perfecty sinusoidal. More great words: most regions have either diurnal (daily) or semi-diurnal (twice-daily) tides. Oh, it gets even worse! Usually, if you are in a place with semi-diurnal tides, one high tide is higher than the other, and one low tide is lower than the other! So you get: high high water, low high water, high low water, and low low water. ARGH!!!!!!
When we were on the Fraser River in BC last summer for a field trip, we were a good 100 km inland on the river, but the river is so huge that we still observe tides there. They were NOT sinusoidal. (Of course, thanks to Fourier, we know we can always represent them as sums of sinusoidal components - but that's another story!) They consisted of a sharp rise to high water and a slow drop to low water. The river nearly ground to a halt just before high water - compared with a speed of about 1m/s when the tide was going out. It was quite something to observe!
