Problem - Swimming in Rising Water

Given an $n \times n$ grid of elevations, find the minimum time $t$ such that you can swim from $(0,0)$ to $(n-1,n-1)$.

At time $t$, you can swim to any cell with elevation $\leq t$. Water rises uniformly across the grid. Example:

grid = [[0,2],
        [1,3]]

At $t=3$, all cells are reachable. Answer: $3$.

This combines Union-Find with time-based activation. Constraints: $n$ from $1$ to $50$, elevations from $0$ to $n^2 - 1$.