Hi,
I'm in zone 7 Washington DC area. My garden has been planted. We are getting an incredible amount of continual rain.

When should one be concerned about root rot or other excess water problems?

Can anything be done?

Thanks

There is really nothing you can do now. When the season is over you could add things to loosen the soil if it has too much clay.
But if it is a sandy soil, there shouldn't be much of an issue.