Greyhound is watchable but entirely fanciful.
Tom Hanks is a Captain of a destroyer taking on the u-boat menace. He's fearless, god fearing, modest and faces down hordes of u-boats who are out to get him.
The enemy goads him, sends u-boats to face off with destroyers in 18th century style exchanges and sails around on the surface following his floatilla, in open defiance.
Now for an injection of reality. U-boats did not take on destroyers in surface battles of the kind you see in this film. They did not goad their captains (and records show they took no pleasure in the job they did) nor did they let the destroyer know they were there. This is a fiction and a rather silly one at that. U-boats were not a match for a destroyer, in any sense and made every effort to evade them.
The US in the early days of this war, were hopelessly outclassed by u-boat packs. It took considerable time before they started defending merchant shipping. Their admirality mostly declined advice and help from their more experienced, British counterparts. This predictably made the situation a lot worse. Eventually they did catch up but only after suffering massive losses.
This film is, in short, a fiction. Its entertaining, its well acted, I do like Tom Hanks and the action scenes whilst in many instances ridiculous, are well rendered and frenetic.
So by all means watch Greyhound for pure entertainment but don't expect a "Das Boot" experience by any means. Its worth mentioning too that even Das Boot was not 100% correct but a lot closer to the truth than this film could ever hope to be.
6/10.