The Richmond Whig newspaper carried this article on August 4, 1865 covering the arrival of the Lincoln assassination conspirators to their prison of Fort Jefferson, Dry Tortugas:
What surprised me the most about this article is the claim that, upon reaching the island, the prisoners were relieved at finding it, “not so bad a place as they had supposed,” as it had a “fine sea breeze” and was a “very healthy” place. Nothing could be further from the truth.
Early in his memoirs, Sam Arnold accurately describes the Fort thusly:
“Without exception, it was the most horrible place the eye of man ever rested upon, where day after day, the miserable existence was being dragged out, intermixed with sickness, bodily suffering, want and pinching hunger…”
It would have been a fallacy to think that Fort Jefferson was “healthy” in any sense of the word. Scurvy, malnutrition, diarrhea, and diseases like yellow fever ran rampant. The sick were oftentimes quarantined and only aided by a handful of doctors and nurses. No one enjoyed life on Fort Jefferson. Especially not Dr. Mudd, Edman Spangler, Samuel Arnold, or Michael O’Laughlen.
Soldiers in quarantine on Fort Jefferson 1899
References:
Richmond Whig, 8/4/1865
Memoirs of the Lincoln Conspirators by Michael Kauffman
Fort Jefferson Historical Structures Report
LincolnConspirators.com 
Dave, remember that Arnold was a perpetual pessimist; most of what he wrote at that time, and later, was a litany of anguished complaints and dire predictions. I think the true perception of Fort Jefferson lies somewhere between his version and that of the Whig, among others.
I agree with Tom and also suspect that the beauty of the surroundings must have awed them at first. That’s one of the things that current visitors talk about – the clear seas, the flora and fauna, etc.
Also, they were of a time when disease was all around them in their home locations, so it wasn’t new to them. My first thought would be of the humidity problem, but these guys were from Maryland where the humidity in July can outdo the Dry Tortugas – and Maryland doesn’t enjoy the sea breezes!
I’m sure their first impressions changed quickly; but after being onboard ships for about a week Ft. Jefferson probably looked pretty good.