昨夜、雑学王(http://www.tv-asahi.co.jp/zatsugaku/)の特番を観まして。
"セーラー服の襟元がV字に大きく開いている"のは、水夫のヒトが海に落ちて溺れそうになった時に自ら引き裂き、脱ぎ捨て、泳ぎ易くする為らしいです。

Last night, I watched the "Trivia King"(http://www.tv-asahi.co.jp/zatsugaku/) TV program.
Apparently "the v-shaped opening at the front collar of a sailor uniform" was designed for seamen who might fall overboard and be at risk of drowning. It's meant so that they can easily tear off and discard their shirts so that they might more easily swim.