A land breeze is created when the land is cooler than the water such as at night and the surface winds have to be very light. When this happens the air over the water slowly begins to rise, as the air begins to rise the air over the surface of the ocean has to be replaced, this is done by drawing the air from the land over the water, thus creating a sea breeze. Usually land breezes are light and reach only 5 to 8 knots.
A sea breeze is created when the surface of the land is heated sufficiently to start air rising. As air rises, it is replaced by air from the sea; you have now created a sea breeze. Sea breezes tend to be much stronger and can produce gusty winds as the sun can heat the land to very warm temperatures, thereby creating a significant temperature contrast to the water. Speeds of 10 to 20 knots is not uncommon for a sea breeze.