I would give it a try like that:

1. Generate a heightfield for your map (e.g. with perlin noise)
2. Determine a height which represents sea level
3. Fill all tiles below sea level with your water pool tile

I would give it a try like that:

1. Generate a heightfield for your map (e.g. with perlin noise)
2. Determine a height which represents sea level
3. Fill all tiles below sea level with your water pool tile