DELETE FROM tbl_boat, tbl_qualifications
WHERE tbl_boat.BT_ID = '$bt_id' AND tbl_boat.BT_ID = tbl_qualifications.BT_ID;

DELETE FROM tbl_boat
INNER JOIN tbl_qualifications
WHERE tbl_boat.BT_ID = '$bt_id' AND tbl_boat.BT_ID = tbl_qualifications.BT_ID;

DELETE tbl_boat, tbl_qualifications
FROM
    tbl_boat
    INNER JOIN tbl_qualifications USING (BT_ID)
WHERE
    tbl_boat.BT_ID = '$bt_id'

DELETE tbl_boat, tbl_qualifications FROM tbl_boat, tbl_qualifications
WHERE tbl_boat.BT_ID = '$bt_id' AND tbl_boat.BT_ID = tbl_qualifications.BT_ID;

DELETE a.*, b.*
FROM tbl_boat AS a
INNER JOIN tbl_qualifications AS b
    ON b.BT_ID = a.BT_ID
WHERE tbl_boat.BT_ID = '$bt_id';