I tried hiding under a crippled tanker once
Had 2 escorts circling like sharks
In the end their patience snapped and they began DC drops
So close THEY sank the tanker
With me under it

I have tried that with various degrees of success.

I use ships for cover only to slip out of a convoi because I usally position my uboot between the rows of a convoi to attack it.
But I never hide under a sinking ship, because once I tried to pass under a carrier which, I thought it wasn't going down and in the end it draged me down to about 160m. :damn:

I've hidden under a crippled cruiser before, so I guess it's OK. It's risky, as others have mentioned, as your cover can suddenly "drop in" on you!