If the battery has or is discharged, the correct method is to charge is on a charger. That takes a lot of stress off of the car system components. Idling will not fully charge a battery.
First, before charging, I would check the voltage between each cell. A wide variance could indicate an internal short or shorted cell. Not good. If it has been boiling off water, then you also need to check the charging system.
What ever you do, make sure to check it prior to trying to charge it. They can and will explode. I have the experience to back that statement.
Fortunately, the battery was out of the boat at the time.
Using a weak battery can cause you a multitude of headaches, including starter issues.