Even if a settler factory wont grow above size 6, being on fresh water could be the difference between a 4-turn and a 5-turn setttler factory.
Example: Your city has one grassan wheat, one grassland wine, a forest, two BGs, and two regular grasslands. The wine and the wheat are irrigated, and the grasslands are mined,
If your city is on fresh water, then:
Turn one: Size 5, working the wheat, wine, a grassland, and the two BGs. Makes 6 shields
Food stored: 5, Shields stored: 6
Turn two: Size 5, working the same tiles. City grows and grabs the forest on growth. Makes 8 shields
Food stored: 0, Shields stored: 14
Turn three: Size 6, working all tiles mentioned except the forest. Makes 7 shields.
Food stored: 5, Shields stored: 21
Turn four: Size 6, working the same tiles. City grows and grabs the forest on growth. Makes 9 Shields, settler completes, city at size 5.
Food stored: 0, Shields stored: 0
If it is not on fresh water, then:
Turn one: Size 5, working the wheat, wine, a grassland, and the two BGs. Makes 6 shields
Food stored: 5, Shields stored: 6
Turn two: Size 5, working the same tiles. City grows and grabs the forest on growth. Makes 8 shields
Food stored: 0, Shields stored: 14
Turn three: Size 6, working all tiles mentioned except the forest. Makes 7 shields.
Food stored: 5, Shields stored: 21
Turn four: Size 6, working the same tiles. City does not grow because of lack of fresh water. Makes 7 shields.
Food stored: 10, Shields stored: 28