Why did this movie get made? (possible spoilers)
Based on a novel from the '70s which was set in the Pacific Ocean with an American submarine, there's very little sense in this adaptation. I will mention...
The fighter plane attack is in the book. (Zeroes rather than Me109s, naturally).
More sex and sexual situations in the book (hey, it was written in the '70s and was a cheesy potboiler, naturally there was a fair bit of sex in it.)
To the person who claimed that WWII submarines didn't have radar: Horse puckey. Not only did both British and American submarines have radar from 1942 onward, but by early 1944 most of them had PPI scopes (Plan Position Indicator, aka "the rotating radius on circular display".
Sub attack (that is, the attack on the other sub) is in the book - in fact, as I recall that's when Hardy and Cassidy start wondering if they're going to cause a war, as they know the US doesn't have any diesel subs in the Pacific - but the Russians might...