Narrow seas means: Those seas (bodies of ocean water) which surround the British Isles, especially the English Channel.. It is legal vocabulary; usage is usually formal, institutional, or news-related rather than casual teen talk. There is no real cause for parental concern; it is descriptive vocabulary rather than risky behaviour. If your teen uses it, context will usually make the intent clear. A short, curious question about where they heard it is usually all that is needed to know whether to follow up.