5. Water that suddenly pulls back from the shore
If the sea abruptly retreats — exposing sand, rocks and flapping fish where water just was — you are not watching an odd tide. You are watching the single clearest natural tsunami warning that exists, and you may have only minutes. It’s called drawback: the trough of the tsunami arriving before its crest. People walk out to look at the stranded fish; survivors run uphill. Teach your kids this one sentence and consider this page worth it: if the ocean runs away, you run too — inland and uphill, immediately, and don’t come back for hours. The same applies to any unexplained, fast, sustained change in sea level.
6. The “champagne” foam line that smells wrong
Sea foam is usually harmless — churned-up algae and organic matter, the ocean’s cappuccino. But a thick, persistent, brownish or reddish foam with a chemical or rotten smell can mark a harmful algal bloom (a “red tide” cousin), and on those days the spray itself can set off coughing fits and sting eyes, while shellfish from the area become genuinely dangerous. The tells: discolored water (rust, brown, or pea-green streaks), dead fish along the tideline, and that wrongness in the air. Swim elsewhere, keep dogs out of the foam — dogs are the most frequent victims — and skip the beachside mussels that week.