If you know the Elco (speed boat) is coming, change your course and travel at 1kt as deep as you can. You can usually escape an Elco after it has found you just by changing course, the Elco will normally circle your last known position until backup arrives.
Or you can take a totally different route away from the sinking merchant and bypass the Elco & Destroyer completely.
You should be able to escape.
It will be hard and you may need many course changes.