I thought he'd have to have more sex appeal and a more of a surfer look. Although, that is casting just on physical descriptions and most of the time people will be disappointed based on looks. I'm sure he could act like their Finnick - the one written in the script. It is the most annoying aspect of Books to Films. Everyone already has their "perfect" characters in mind. When they cast Josh Hutcherson as Peeta I about keeled over - he didn't look or seem to be able to act like him based on past performances, but I think he pulled the character off well. I was actually surprised in a good way when they cast Woody Harrelson as Haymitch, but I feel that Woody butchered my Haymitch, so it goes both ways.
On a side note, the same thing happened with this other Book the Movie series franchise in the making - "The Mortal Instruments" - Everyone wanted Alex Pettyfer for the part of the lead male role. I wanted to strangle those people lifeless, because I think the guy is an idiot that can't act. Then they cast Jamie Campbelle Bower and I went "blah" again.. lol.. he looks too... grunge - for the part, but they say he nailed the character, so for now I'll just have to wait and see.