SAN JUAN, P.R. — On an island that is flirting with default, fending off comparisons to Greece and losing its people to the mainland, the biggest problem most people face is something more elemental — one of the worst droughts in Puerto Rico’s history.

There has been so little rain here that two months ago the government was forced to start rationing water on the populous eastern side of the island, including in many San Juan neighborhoods. Carraizo, the major reservoir serving parts of the city, has dropped nearly 18 feet in recent months, shrinking so noticeably that people can now fish off its sandy shores. The last time water rationing was ordered on the island was two decades ago.

For 160,000 residents and businesses on the island, water is turned off for 48 hours and then back on for 24 hours, sending people into a frenzy of water collection. Another 185,000 are going without water in 24-hour cycles, and 10,000 are on a 12-hour rationing plan.

“This is the strictest rationing we’ve ever had,” said Alberto M. Lázaro, the executive president of the Puerto Rico Aqueduct and Sewer Authority. “It’s been raining, though the showers that should drop water in the east coast are dropping the water in the west coast.”