St. Augustine grass is one of the rare plants that is somewhat tolerant of saltwater. Most grasses and plants die when doused with saltwater, but St. Augustine grass is a tough ocean coastal plant that has adapted to levels of salt. That doesn't mean that saltwater is good for it, but it does open the door for weed-killing with saltwater concoctions, with lower risk to the grass. Combining salt with boiling water is an extremely effective weed killing method to get rid of weeds in St. Augustine grass.
Fill a kettle with tap water and heat it until boiling. If you don't want to put salt in your kettle, you can transfer the boiling water to a saucepan before adding it, or simply boil it in the pan.
Mix in about one cup of salt for every two cups of water. If your kettle holds six cups, add three cups of salt, for example.
Pour the mixture into a hand spray bottle or a small-mouthed pitcher to limit the amount that comes out at one time. If you're pouring from a saucepan into a spray bottle, a funnel will be helpful to minimize spilling.
Spray or pour the hot saltwater as closely onto the unwanted weeds as you can. If using a spray bottle, slip on garden gloves to protect your hands from the heat. Even though your St. Augustine grass is salt-tolerant, try to keep the saltwater directly on the weeds, to avoid damage to the grass.
Wait several hours to allow the salt to reach any deep roots, then pull up the weeds. They should be dehydrated and dying, so they should be easy to pull up by the roots.