public class Assent
{
    //...
    public virtual int Id { get; set; }
    //...
    public virtual IList<WaterUsePurposeSet> WaterUsePurposes { get; set; } // список
    //...
}

public class WaterUsePurposeSet
{
    public virtual int Id { get; set; }
    public virtual WaterUsePurposeType Type { get; set; }
}

public class WaterUsePurposeType
{
    public virtual int Id { get; set; }
    public virtual string Name { get; set; }
}

using (ISession session = sessionFactory.OpenSession())
{
    using (ITransaction tx = session.BeginTransaction())
    {
        IList<Assent> tempList = null;
        foreach (WaterUsePurposeType t in types)
        {
            IQueryOver<Assent, WaterUsePurposeSet> quer1 =
            session.QueryOver<Assent>()
                   .JoinQueryOver<WaterUsePurposeSet>(c => c.WaterUsePurposes)
                   .Where(k => k.Type.Id == t.Id);
            tempList = quer1.List();
            assents.AddRange(tempList.Where(asss => !assents.Contains(asss)));
        }
        tx.Commit();
    }
}