To deck change or no?

Wear the same outfit from start to finish, and under your wetsuit.

The only races that have change tents are full-distance Ironmans, and nudity is against the rules so I'm not sure how you would change in transition. I've seen some people swim in bathing suit and throw on shorts/top in T1, but the MUCH more efficient way is to do what you do for shorter races.

For the ocean swims I've done in Asia, you usually run through a fresh-water shower or hose as you exit the water. I don't know if this is common in the US......if you pull the wetsuit down to your waist as you exit the water, you could pretty much get rinsed off. Or squeeze some water over yourself in transition. Most of my tris have been ocean swims, though without a wetsuit. Haven't had any problems particular to salt water. (Though it makes racing with road rash pretty painful!) I've done them all (including the HIM, which was in a lake with wetsuit) in a tri top and shorts, or a one-piece. The two piece outfit is somewhat more convenient if you need to use the porta-potty during the race, which is more likely in a longer event. I prefer the one-piece as it's more comfortable on the swim, and wear it up to Oly distance. You do want to make sure that your shorts are confortable for the long haul, and you've used Body Glide where needed.  Wouldn't advise pulling bike or running shorts over a wet swim suit--that's asking for chafing.

Tri shorts, tri top, sports bra under wetsuit from start to finish. In all the HIMs I've done or been to, I've never seen anyone change in transition. There will be no tent and change under a towel...on wet body and tight clothes will be challenging.

Tri shorts do well and are designed to wear whole race. Ideally you will try out your wardrobe in s/b brick prior to your race and find it out.

I too worried about salt and chafing in longer events but haven't had issues other then wetsuit chaffing in salt water.

One or two piece tri-suit under the wetsuit.  Lots of BodyGlide and Chamois butter (esp. for salt water).  Most races don't have hoses, but you can easily spray down with a water bottle while on the bike if needed.

Here in Cali lots of 70.3s have saltwater swims, and I've never had an issue on the bike.