Don't panic. Unscrew the crown slightly to allow any moisture to escape, and place the watch in a container with silica gel packets if you have them -- that is the most effective way to draw out moisture. Keep it away from heat sources. Skip the rice trick -- it is more urban legend than fix.
Once the moisture has cleared, we recommend sending the watch in for a service or having a local watchmaker service and reseal to prevent it from happening again.