Use of conjunctions

Following the principles described by John Ross in his famous dissertation Infinite Syntax, an adverb can be moved to the beginning of coordinated clauses only if it was in both clauses before being moved. In your example,

... in the sea, where no one should be found yet each one must be keen as they play hide-and-seek.

"where" represents the "there" which occurs in each of the two coordinated clauses:

no one should be found there

each one must be keen as they play hide-and-seek there

In the sea, [where no one should be found yet each one must be keen as they play hide-and-seek].

The bracketed element is a supplementary (non-defining) relative clause in which the preposition "where" has the PP "in the sea" as antecedent.

Supplementary relatives are not modifiers but separate units of information, non-constituents, that have a semantic 'anchor'. Here the anchor is the PP "in the sea".

Note that "yet" is not a conjunction, but a connective adverb with a concessive meaning.