Simplify the following text for a Grade 13 native English-speaking person.
Original: Saint Thomas is an island in the Caribbean Sea and with the islands of Saint John , Saint Croix , and Water Island a county and constituent district of the United States Virgin Islands ( USVI ) , an unincorporated territory of the United States .
Simplified: Saint Thomas is an island in the Caribbean Sea , a county and constituent district of the United States Virgin Islands ( USVI ) , an unincorporated territory of the United States .