BioShock was the first FPS I ever played, and I thought it wasn't very difficult (I played on Normal). However, I have played a lot of survival horrors, and this game feels more like a mix of those 2 genres than a straight-up shooter. It's not about how much you pump the enemies full of bullets, but how you use the environment to your advantage. Even Big Daddies are pretty easy if you do the ole "stun 'em and whack 'em" (in my case, whacking was actually moving to a safe distance and then using the mine thrower or another strong weapon, lol).

But I imagine the game is much more difficult on Hard, and that's why they let you change difficulties mid-game. So...I'm not really sure what you're complaining about? Just never do your first playthrough of a game on Hard, unless people have told you that it's a really easy game.