I've noticed this, and I have to agree. While the idea of Simba diving in to save Kiara was nice, considering the tension they went through...it makes absolutely no sense that Kovu would just sit there and do nothing. I can understand the argument of him not wanting to get get in the way...but surely once Kiara and Simba were safe he would have done SOMETHING to help them back up to safety.
Even after he fell in love with Kiara, Kovu was never the type to sit on the sidelines and watch like that. When Zira ambushed Simba earlier in the movie, he dived striaght in, trying to help Simba...why wouldn't he do so now, for the one he loved?
And I agree with Moka's idea too...that sounds awesome! Too bad the writers didn't think of that.