I'd be more worried about what might be living in it, than what the salt has done, plus if you don't know the age of the beam, I'd definitely want a pro to check it for inner rot, etc.

Any critters would smell as they died. That's one concern. As for inside, I would be cutting into pieces so would know right away what condition it was in. Apart fronm that, dry rot does not occurr in salt water, only fresh.

There's got to be a lumber salvage company in your area. I'd be looking for a phone number or address and ask them.
Pulling timbers out of water both fresh and salt has been going on for a long time. Loads shift, boats sink, timber sometimes gets lost.
There was a tv show about it not too long back on either Discovery Channel or the History Channel.
You're best bet is to talk to someone whose been in the business for awhile and get their take on what to do.

Fully submerged timers are often just fine after drying out.

There is a huge market for old growth wood that has sunk in rivers and can now be retrieved and milled.

If this is a piece of old timber, it will be better than anything (except salvage) you could buy today - tighter grain, older growth, etc. The one thing to look out for is embedded pieces of metal you can't see. These can ruin a saw blade, so using a carbide blade or checking with a nail finder/metal detector before cutting is the way to go.