満ち潮で水没する砂浜でキャンプを張るのは、実に愚かな行為です。

、。

Camping on a beach that gets submerged at high tide is truly a foolish thing to do.

Literal

High-tide [with-で] submerge sand-beach [at-で] camp [object-を] setting-up [topic-は], truly foolish act [copula-です].

満ち潮 ('high tide, rising tide') and 水没する ('to be submerged') are specific geographic/maritime vocabulary. キャンプを張る ('to set up camp,' literally 'stretch a camp') is the standard idiom. 実に愚かな行為です ('is truly a foolish act') is formal and judgmental. The sentence reads like a safety warning with a withering editorial tone.