<asp:CustomValidator ID="CustomValidator2" runat="server"
            EnableClientScript="true" OnServerValidate="ins_server"  ClientValidationFunction ="ins_client"
            ErrorMessage="CustomValidator"> * Insurance dates not valid without supporting attached document</asp:CustomValidator>

protected void ins_server(object source, ServerValidateEventArgs args)
{
    //new user
    if (PageMode == PageModes.NewVessel)
    {
        if (fuAttachment.HasFile && datetimepickerinsend != null && datetimepickerinsstart != null)
        {
            args.IsValid = true;
        }

        else
        {
            args.IsValid = false;
        }
    }

    //existing user
    if (PageMode == PageModes.EditVessel)
    {
        args.IsValid = true;
    }
}

//declerations
var insurancestart;
var insuranceend;
var filesattached;

function ins_client(sender, e) {
    if (pagemode == 'EditVessel') {
        e.IsValid = true;
    }

    if (pagemode == 'NewVessel') {
        if (insurancestart !== '' && insuranceend !== '' && filesattached > 0) {
            e.IsValid = true;
        }
        else {
            e.IsValid = false;
        }
    }
}

<script type="text/javascript">

     var pagemode;
     $(document).ready(function () {
         // removed var to give global scope
         pagemode = '<%=this.PageMode.ToString()%>';
     });
</script>