Ocean
The Ocean or Sea field was shown briefly in Duelist Kingdom, used by Mako Tsunami. It powered up WATER monsters as well as hiding them under the water. Other monsters could not occupy it. It is based on the "Umi" Field Spell Card.
In early video games the field can be changed to Ocean, by activating "Umi", which granted the ATK and DEF changing effects of "Umi" to the affected area.
