Historical clue formulations

• School houses? — 2026-02-28
• Coral-based ecosystems — 2022-04-06
• Underwater ecosystems — 2021-06-23
• Prime snorkeling spots — 2021-02-25
• Some barriers to entry in the shipping business? — 2021-01-13
• Perils for mariners — 2020-02-22
• Some endangered ecosystems — 2019-03-17
• Water hazards — 2018-11-21
• Navigation hazards — 2018-06-06
• Locales for many schools — 2015-05-01
• Navigational hazards — 2015-03-26
• Settings of many schools — 2012-09-22
• Hull scrapers — 2008-12-28
• Marine hazards — 2007-10-10
• Coral groups — 2006-08-24
• Snorkeling destinations — 2006-07-03
• Sailing hazards — 2004-08-31
• Coral ridges — 2004-07-12
• Marine dangers — 2003-05-07
• Shortens, as a sail — 2002-07-19
• Snorkeling areas — 2002-03-07
• Coral assemblies — 1998-06-21