What appears on the surface is merely the tip of the iceberg.
Literal
Surface [to-に] appearing [topic-のは] iceberg [possessive-の] tip [to-に] only [copula-過ぎない].
~に過ぎない means 'is nothing more than, is merely.' 表面に現れているのは nominalizes 'what appears on the surface.' 氷山の先端 means 'the tip of the iceberg' — a borrowed metaphor that works naturally in Japanese.