Kata Yai is the main Kata beach, a kilometre-long crescent of pale sand backed by casuarina trees and budget-to-midrange hotels. In the calm season (November to April) the water is clear, swimmable, and shallow enough for kids; the rest of the year, the waves pick up enough to support a small but serious surf scene.
Sun loungers and umbrellas line the front (300–400 baht for the day); the south end near the rocks is quieter and shadier. Watch the red flags during the monsoon — rip currents have killed swimmers here.