Why? don't you have a height hold? If the fire doesn't go out and you have to ditch - better to do that quickly from 100' than all the way from 500'.
As a standard exercise I give an engine fire at 200' in the cruise (IMC or night) over the water so the pilot can trans down whilst dealing with the fire. If it goes out then trans up, if it doesn't you are in a perfect place to ditch from 50'. If you wait for the prescribed time (30 secs on the SK) after the second bottle goes in, it will still take you a further 20 secs to get the aircraft on the water, even from 50'.