Ko Tao, Thailand

With a population of just 1,382, this tiny Thai island is almost entirely devoted to tourism - particularly scuba diving. The economy is singularly focused on catering to the influx of over 132,000 visitors each year, which has put a major strain on the island's infrastructure. Shortages of electricity and fresh water are common, and the disposal of the 45,000 tons of solid waste produced annually is a persistent headache.

The island's popularity has also come at a cost to its fragile environment. Overuse of dive sites has damaged coral reefs, and the growing abundance of invasive species like crown-of-thorns starfish threatens the marine ecosystem. Authorities have attempted to address these issues, but their efforts have yet to bear fruit.

If you can overlook the bureaucratic and environmental challenges, Ko Tao offers world-class scuba diving at rock-bottom prices - making it a prime destination for budget-conscious underwater enthusiasts.