The sooner you can get out the better. You want to get it out before it has a chance to spread and bond with the EPS core of your board. How much worse it can get depends on how much water the board takes in and how warm the temps are.
Think about boiling water in a hot water kettle. How do you know when the water has boiled? The whistle goes off right? When the water is heated it becomes a gas-steam-that expands. If you have water in your board and the board is exposed to heat-sun-with the vent plug closed, the pressure has to go somewhere right?