Barry Newman had a brief vogue as an action hero in the early 70s with films like Vanishing Point, The Salzburg Connection and this. It's based on an Alistair MacLean novel, during a period when a series of such adaptations struggled at the US box office - When Eight Bells Toll, Puppet on a Chain.
This is the wisdom of hindsight, of course, but I think you can tell why - for starters Newman isn't an automatic hero who you accept and enjoy as a tough guy. That gives the piece some novelty, he's excellent in the dramatic moments at the end, he's a believable family man - but not so much as a brilliant car driver, or someone handsome enough to turn Suzy Kendall into an ally even after he's kidnapped her, and/or someone to punch out various henchmen.
There's a typical MacLean twist that involves us not knowing what hero Newman is up to until the two thirds mark - thinking he's a baddy then discovering he's a goodie. He had Richard Burton do that in Where Eagles Dare. The thing is, for most of Dare we thought Burton was a goodie, then briefly thought he was a baddy, then realised he was a goody. Also we had Clint Eastwood as an audience surrogate.
Here Newman seems to be dodgy - he starts a fight, shoots a cop... it doesn't make him very sympathetic. We guess that there's some agenda because in the opening scene we see him hear a girlfriend and friend be killed... but maybe it would have been better if we'd known all along, we could have been more emotionally invested in him. Of course it is a nice reveal at the end that his young son was in the plane... but it's very late in the day. Maybe the best thing to do would have been tell the story from Suzy Kendall's point of view - she could have been the audience surrogate (like Audrey Hepburn in Charade).
On the topic of Kendall, she wasn't the best actress in the world but her part needed to be bigger. She's just this good looking frightened thing - we never get a sense of growing attraction toward (or indeed any feelings about) Newman. The involvement of Ray McNally - as Kendall's father -felt under-developed too.
Positives include stylish photography and music, some excellent support performances (including Ben Kingsley as a scary henchman, and Dolph Sweet). And the final confrontation on the ocean floor was effective. Just don't stress too much about logic.