The night scenes are filmed very well but you must see them in wide-screen format or letter-boxed. For example, close scenes inside the ship at night are well filmed and staged, but literally won't make sense if not viewed letter-boxed. The outdoor sea scenes (almost all dark) are awesome, probably better than if they had been computer-generated. But again, the movements of the ships will not make sense if viewed other than letterbox. The story is largely told through movement- of ships, or of men moving around in the dark. I mention this to help any potential viewer.
Cooper gives a compelling, desperate performance that makes you really want to watch what happens to his character, Capt. Patch. Also, this is probably the most believable performance by Heston as an ordinary, non-historical, non sci-fi character. I was pleasantly surprised by both Cooper's and Heston's performances. Both performances are essentially very modern-style screen acting, and are not dated after these many years.
The supporting roles are undeveloped and one-dimensional, including the role so forcefully played by the young Richard Harris. Its a shame the great Virginia McKenna is so underused here. She could have made the film appeal to a broader-based audience. The other supporting actors (Alexander Knox at his most wooden) don't add to the film.
Even if you don't like seafaring stories, watch this for the acting by the two stars, and for the marvelous night cinematography. Film students and buffs should look carefully at the night scenes inside the ship and on the docks. You will see true night cinema work, with perfectly set light meters and minimalist lighting. In other words: nighttime lighting and camera artistry, not gimmicks.