It isn't a poor choice at all IMO. It's a smart choice and makes perfect sense to me. And here's why I say that. People would expect him to choose those he's known longer or is closer to. And if he's faking his death to get this really bad guy (Ron Saris), then H probably thinks "well, he'll go to the ones I'm closest with to find out if its true about my death". That's why he didn't choose Calleigh or Eric to help him because it would be way too obvious to the criminals. And I also think that Horatio sees a lot of his younger self in Ryan. Ryan is sort of (for lack of a better term) Horatio's "apprentice". :lol:
Plus, with Eric you have the head injury. Eric isn't all there right now. He's been making mistakes left and right due to his brain injury. I think that Horatio knows this inspite of Calleigh covering for Eric. I think Horatio knows pretty much everything about his team. :lol: He couldn't risk Eric getting himself hurt worse because his injury caused him to make a mistake.
And Calleigh, well... she's his second in command, so that would just be too obvious. Plus, Ryan has done undercover for H before (trying to find the mole) and of course Jake is a UC, so he'd be able to help Horatio quite a bit. Sometimes to get the job done without the villians figuring it out, you have to go with the least obvious choices for help.
Horatio needs (as bad as it sounds) those who are closer to him to believe he's dead in order to better sell it to the bad guys. Ryan's gambling past is actually possibly helpful here. Ryan's gambling was poker I believe (at least one time it was), so therefore it only stands to reason that Ryan could possibly be good at bluffing (all good poker players are), right? So, Ryan could put on his "poker face" in this situation. He could better act like he thinks Horatio's dead.