if (chemexist == false) // conditionally creating WtrChem record
    {
      WtrChem wc = new WtrChem();
      wc.Contact = "John Smith";
      ..
      wc.Phone = ("800-888-9988");
      Db.WtrChem.Add(wc);
      Db.SaveChanges();
    }


    WtrChemDetail psw = new WtrChemDetail (); // creating details record for WtrChem
    psw.Comments = comment;
    ..
    ..
    Db.WtrChemDetail.Add(psw);
    Db.SaveChanges();

// Get the item. Include makes sure that you get the referenced detail as well.
WtrChem wc = Db.WtrChem.Include(x => x.Detail).SingleOrDefault();

if (wc == null) // creating WtrChem record
{
   // If it wasn't found, create and add
   wc = new WtrChem();
   Db.WtrChem.Add(wc);
}

wc.Contact = "John Smith";
..
wc.Phone = ("800-888-9988");

// Deal with the
WtrChemDetail psw = new WtrChemDetail (); // creating details record for WtrChem
psw.Comments = comment;
wc.Detail = psw;

Db.SaveChanges();

using System.Data.Entity;