You couldn't keep a secret like that forever (I saved my dog while watching a kid drown)...I'd eventually find out.
And, just because you are right there doing the swimming, that doesn't mean witnesses aren't around.
Maybe witnesses that for whatever reason (handicapped, elderly, etc...) can't swim well enough to help.
"Yes, we saw what happened. This guy jumped in the lake, we thought he was going to save the kid, but he went after
a dog instead".
I'd rather he saved the dog out of love, than save your kid out of fear.