ok the yogscast had a mod spotlight about mo creatures right? remeber that they said "notch didnt want a shark mob because it is too fast for players" well now think about endermen what do you think now notch?
You don't see the difference between a fast shark and endermen?
In water, you have limited mobility and limited abilty to modify the world to your advantage. So sharks become these torpedoes of doom if you are in the water that will just rush over and kill you unprovoked. This arbitrarily makes boat travel deadly for no good reason.
Endermen are peaceful, and only attack when you look at them. Their attack is entirely your fault. They are meant to start from the distance, and so the speed is needed to close in. You are fighting them on ground, and hence are not placed at an additonal disadvantage.