Maximise Islands

You are given a grid with $N$ rows (numbered $1$ through $N$) and $M$ columns (numbered $1$ through $M$). Each cell of this grid contains either water or land. The initial state of the grid is described by strings $S_1, S_2, \ldots, S_N$; for each valid $i$ and $j$, the $j$-th character of $S_i$ is '.' if the cell in the $i$-th row and $j$-th column initially contains water or '*' if this cell ini