Question: Who exactly is Linarcos?
Answer: He's the representative of the owner of the ship.
Question: Why did Jack Albertson hate working with Shelley Winters when making this movie?
Answer: There's nothing specific reported about them, but other actors have also expressed disliking working with Shelley Winters. Stella Stevens, who was also in Poseidon, mentioned not getting along with Winters on another movie they made together because she was drinking at the time. Winters had a rather ditzy and brash personality that annoyed some people, which is probably what Albertson was referring to.
Moreover, Albertson once said, "The happiest moment making the movie for me was when she died."
Answer: Because he doesn't boil, there's no suggestion that the water is that hot. After Ackers falls in, and while Rogo tries to save him, there are two large explosions in the shaft; we can presume that, while Rogo was able to escape, Ackers was injured/trapped/otherwise incapacitated below the surface, and drowned.