Okay, after some discussion, this is what I have to offer.
Bob saw Meg swimming-- Meg was swimming, and Bob saw it, plain and simple.
Bob saw that Meg was swimming (I prefer it with a "that")--It can probably be used in situations like, Bob is investigating Meg's death, after some time (evidence was gathered), Bob could see in his mind/head that Meg was swimming when she was killed. Bob saw that Meg was swimming.
This post can be total nonsense, and if it really is, I am really sorry, I am just trying to help, and I am sure others will make sure I know how wrong I am.
It is acceptable as you've written it. However, I would use "that."
I've referred to my trusty HarperCollins Concise Handbook for Writers. they devote a section to ommitting that. The rule is simple. The subordinating conjunction that can often be omitted, but not if omitting it makes the sentence hard to read.
In your sentence it would be silly to think that Bob was a swimming when he saw Meg, but if someone might misread the sentence to that end, you would need that . . . was.
Otherwise, like Nichec, it doesn't sound right to me without that. I will guarantee that you would not omit "que" [that] in the same sentence translated into French.
Yes, Panj the rule and its examples which I read only used the condition that you needed that if the sentence could be misunderstood. I assumed that there is one person somewhere who might become confused. When I get into matters that might be confused I usually go to my cat, Dora. However, on this one I figured that someone from the Forum would come forward.
Dora and I communicate telepathically Remember that next time she looks asleep ...
Bob saw Meg swimming.
I was suggesting earlier that this could be equivalent to Bob saw Meg when Bob was swimming.
As Harry Batt gently pointed out, it is normally understood that Meg was swimming when Bob saw her.
But my other sentences make it clear (to me at least) that there is nothing inherent in the sentence structure to prevent that alternative understanding. That's why I picked the two middle sentences of the group. Here they are again: Bob saw Meg on her way to the park.
I don't know anything about what Bob was doing, but Meg was going to the park. Bob saw Meg on his way to the park.
I know that Bob was going to the park, I have no idea what Meg was doing.
And what about the difference between these two (I can't cope with the labels, sorry). Bob saw Meg swimming. Bob saw that Meg was swimming.
I find this difficult to explain.
The first sentence focusses on Meg and the fact that Bob saw her.
Bob saw Meg.
The fact that Meg was swimming at the time is rather incidental - it is an additional, non-essential part of the sentence.
The second sentence has a lot more focus on the fact that Meg was swimming, almost as if Bob was surprised by this.
I think I have overstated the difference, but I am convinced it exists.