The part of an iceberg hidden underwater is far larger than the part above the surface.
Literal
Iceberg [possessive-の] water [in-に] hidden part [topic-は] water-surface above [to-に] appearing part [than-よりも] far big.
A factual comparison using よりも ('than'). 水に隠れている部分 and 水面上に表れている部分 are parallel relative clauses. はるかに means 'by far.' A clear example of comparative structure.