The Gulf of Thailand was once the edge of Europe’s known world.
The Magnus Sinus – or Great Gulf – was a feature on maps before the Age of Discovery. Initially, the great Ptolemy misinterpreted earlier accounts from seafarers and had the Great Gulf enclosed on the East by the “Dragon’s Tail Peninsular” – what was assumed to be a continent-sized body that filled in all the space between the Islands of Indonesia and Indochina as though the South China Sea was all land. Thats where Christopher Columbus thought he was going when he set off to the Americas. The Isthmus of Kra – the neck of land we’re going down, somehow was thought to not exist at all, yet they knew about the Gulf of Thailand – odd!
The Gulf is an interesting body of water: Its relatively shallow – only 85 metres at its deepest – and with so many large rivers depositing into it, there is a lot of sediment and its waters are not particularly salty. The salt waters of the South China Sea flow in only at around 50 metres and below.
The occasional limestone karst bluffs continue into the sea, creating islands and seamonts against which corals cling, or at least used to. Only 5% of the 120 square kilometres of coral reefs are now considered ‘fertile’. This is declining, particularly after a massive coral bleaching event in 2010. So if you want to go diving in Thailand, do it soon, and consider diving on the other side of the Ithsmus – I’ve personally dived the Similan Islands and it was amazing (but that was before 2010).