I think this might be a duplicate report, but I can't find the original one. I've had the AI do this many times - it will attack as if it's going to play a blocker after the attack (if it doesn't it will die), but then it will play something that comes into play tapped (like Tormented Hero) or something that can't attack or block (like Vigean Hydropon). I also think it's not fully aware that the Hydropon can't attack or block. I think, but I'm not sure, I've seen it also make this mistake with regular "can't block" creatures but can't confirm it atm (for cip tapped and hydropon I'm sure, it happens a lot).