One of Shatner's points in going over that painful episode again is that until you're staring a situation like that in the face, you never really know how you'll react. Over the years, he's asked himself over and over again the same question, why didn't he dive in after her? Why did he call 911 first?
Putting aside the fact that, as anyone who's heard the tape of his call can easily confirm, he was in full-on freak out mode, one thing that stuck in his mind, but really didn't pick up on consciously for a long time, was that the water in the pool was perfectly still.
In other words, she'd been down there long enough that there were no remaining waves from her falling in.
In other words, in the back of his mind, he already knew she was dead and had been for quite a while, and diving in after her wasn't going to make a lick of difference. Which probably goes along ways towards explaining his total freak-out; not only was his beloved wife dead, but he was completely and utterly powerless to do anything about it.
When faced with that kind of conflict, it is a perfectly normal reaction to freeze up.
If anything, he should be credited with having at least enough rational capacity to call 911.