I hate it when..

Punch him in the face. Push his head in the sand, then do push-ups on his back.

or ignore him.

"Wow, you talk a good game for a guy standing on a beach..."

x2

This.  Then kick sand on the d-bag.

"It's only 1/2 a mile, you can do it!!!"

I'd imagine that can be said in both a positive, encouraging tone, and a tone that deserves a b**ch slap.

You are right, but trust me this was a tone that would make you want b**ch slap! I wish I had!!

Just ignore those kind of people, they simply do not have a full understanding of what it takes to compete in triathlon.  Or maybe they do and they are just arrogant. 

If you're going to yell, "you're almost there"  Please, make sure the athlete is indeed almost there.

It is really hard to have an impact on someone who does not understand.  You could always try for sympathy and confusion if you wanted to actually try to change the individual's perspective, hard to do.  I hear that a half mile swim is harder when one has the flu, is recovering from food poisoning, has asthma, is phobic of open water... fill in any reason.  Followed by, it is amazing that the athlete is doing so well given what he/she has had to go through just to get into the water/ to the starting line, etc.

"Almost" is a very subjective word. Better off with "keep it up" or "nice job".

X2

Very dirty look! then when He says another stupid word. Hit em twice.

"Then why aren't you out there?"

If responds with "Oh, I'm tapering" or some BS, then say, well, if you are truly a triathlete, then you know what it is like to work hard for whatever distance. Remember, Olympians do sprints, too.

And then mutter, "stupid bastage" loudly enough for everyone around to chuckle at the idiot.