Ah, yes, that scene. I am of two minds regarding that scene.
Siamesefan:
As it turns out, Tintin will need the Captain. But he couldn't know that ... And if he did believe that, why not just tell Haddock so?
That's the part that I find intriguing. As you say, it isn't in Tintin's character to be asking for help--he's normally the team leader, the take-charge one. He's proven himself to be a more-than-capable solo adventurer so many times before. In addition, he's been trying to convince the Captain that he'll be fine, that everything will be fine, and that they'll succeed in their rescue mission. If Tintin was to ask for help, it would be a sign of his own confidence fading--something that the Captain (who is already concerned for him) would likely pick up on--and that could go two ways: either the Captain would be worried enough to go along with him as planned, or he just might be worried enough to drag Tintin back to Marlinspike for his own safety. Because if Tintin is worried,
everyone should be worried, which may have been Tintin's motivation for not asking the Captain to come along directly.
Siamesefan:
but when Haddock decides to turn back, Tintin tricks him into coming further. By getting him drunk.
...And that's the part I don't like. Not so much the trick as it was getting him drunk--ever since
Crab, Tintin has been the one to try to keep the Captain's drinking in moderation, and seeing him use the Captain's biggest vice/weakness against him was... a bit underhanded, especially when you consider that Allan, the Captain's archnemesis, had been using the Captain's alcoholism against him prior/during
Crab and tried to do again in
Red Sea Sharks.
Of course, Tintin later redeems himself by refusing to cut the Captain's lifeline when he orders him to, so it's all good. That more than made up for that trickery scene!