Well, it depends upon the temperature of the water and the temperature of the land supporting the ice. However, land is usually transferring heat by conduction, while water can also transfer heat by convection. Also, the salt in sea water depress the freezing point.

Well, it depends upon the temperature of the water and the temperature of the land supporting the ice. However, land is usually transferring heat by conduction, while water can also transfer heat by convection, so it should melt faster in the ocean generally. Also, the salt in sea water depresses the freezing point.