I believe he did one full tour on a ship safely out at sea in the general neighborhood of Vietnam.
He'd been activated for two years after he graduated NROTC. He put in for duty in Paris but didn't get it. At the time he volunteered for the Swift Boats, that was cake duty, cruising up and down the SVN coast looking for nighttime VC smuggling by sea, of which there turned out to be exactly ZERO. Before he reported for Swift Boat duty, the Navy had changed their mission to brown-water Riverine patrol, which must have come as a bit of a shock to him when he found out.
I'll give "Pin Prick" Kerry this--he took the lemon and made the best lemonade he could out of it.
BTW, my dates and facts on the Swift Boat force come from a couple books about the VN-era "Brown Water Navy" that were published before Pin Prick became a POTUS candidate or even a major US senator. He might be mentioned in passing in one of them (and might not, I don't remember) and I'm sure he isn't mentioned at all in the other one.