The Moon's gravity pulls on the Earth's oceans, causing them to bulge out on the side of the Earth facing the Moon. This bulge of water is called a high tide. On the opposite side of the Earth, the water also bulges out slightly, away from the Moon. This is also a high tide.

During a "supermoon", the Moon is at its closest point to the Earth in its orbit. This means that the Moon's gravity is stronger than usual, and the tides are higher than usual. This effect of the Sun on tides is similar to, but weaker than, that of the Moon

The difference in sea level between high tide and low tide is called the tidal range. During a supermoon, the tidal range can be up to 40% higher than normal. This can cause flooding in coastal areas, especially if the high tide coincides with a storm surge.

In addition to the supermoon's effect on tides, it can also cause other coastal phenomena, such as stronger currents and riptides. Swimmers and boaters should be aware of these hazards when planning their activities during a supermoon.