If it was 0.0, that is what is called mean low water, not necessarily low tide, but the average medium that they use for charting marine charts. So if it says the high hightide would be 3.0 and the low low tide would be -1.0 the difference would be 4 feet. Usually the high high is higher than the low low is low, because of this mean low water. There are usually 4 tides a day, seperated by a little less than 6 hours apart, like 15 minutes less, per tide, for a moon day of 23 hours, and a sun day of 24 hours. There is a high high and a lower high, and a low low and a not so low low. You have to watch that too. The moon has more pull on the tide, and the sun has some, but not as much. When the moon gets in phase with the sun, and rises and sets with the sun, and both go around pretty much together, which happens about 1 time every 45 days or so, it is called a Spring tide, and tides are stronger currents and higher and lower tides. Weak tides when the moon and sun are far apart, are called neap tides, and usually the 4 tide day is now almost a 2 tide day when it is a neap tide, and the tidal difference isnt as much as normal. This also happens about every 45 days. Also when a cold front or strong winds blow offshore and teams with a spring low, it is extremely low tide. And the opposite, when a hurricane blows on shore, and it is a spring high, it is very high. Just remember the tides are about an hour later each day, as the day before. So they are very predictable, but you have to know the variable which can effect the heights too. If it says low tide is 0.2, that means it is 2/10ths of a foot above mean low. -0.2 means 2/10ths below mean low. high is usually without a +, but low usually has a -, but not always.