As far as sinking submarines wwas concerned, from the USN perspective may I suggest a look at https://www.ibiblio.org/hyperwar/USN/rep/ASW-51/ASW-PART-II.html which discusses antisubmarine measures and effectiveness. For ascribing some sort of credit, you will find that reports of attacks on submarines by surface craft were evaluated as:
A. Known sunk;
B. Probably sunk;
C. Probably damaged, possibly sunk;
D. Probably damaged;
E. Probably slightly damaged;
F. Insufficient evidence of damage;
G. No damage;
H. Presence of submarine uncertain;
I. Target attacked not a submarine; and
J. Insufficient evidence to assess.
In practice, this list was truncated into groups thusly:
Sunk A, B;
Damaged C, D, E;
Undamaged F, G; and
Non-submarine H, I, J
Based upon after action reports, every attack was evaluated and rated as one of the above.
Interestingly, as this document points out, in its appendix one can find that the A-B class evaluations correlated well with actual U-Boat losses, and, even, in fact, in the half of the war tended to be a little conservative in estimates of actual German losses compared to the actual losses.
Of course, one should not mistakes movies, made for entertainment and profit purposes, with history.
