In UT, it is stated somewhere in the tale of Tar-Aldarion and Erendis that rule is passed on to the oldest child (boy or girl) of the previous king, and to the oldest next of kin from there. So probably Isildur himself, then his oldest child would inherit the throne.

On a different note, where was he when he was attacked? 

Forgive me if this is a stupid question.

My favorite king of Gondor is definitely Hyarmdacil ii. He was a fantastic brain, warrior, and politician. He conquered the enemies of Gondor with his skillful armies, thus assuring, the life of that realm.

What about Turgon ii? He did alot to prepare Gondor for the Great Siege of Gondor. Henneth Annun was a excellent hideout were the warriors of Gondor could ambush and destroy travelling companies of Uruks or Haradrim. The camp was constantly manned by rangers and even, by Faramir at sometimes. Also Turgon did much to recruit the men of Lamedon and Langstrand, which seriously helped to bolster Gondor's weakened manpower. Turgon probably realized that Gondor would be assaulted by legions of Gorthaur's orcs and other grunts. Khand probably started pushing further into Harondor during the dotage of Turgon. Turgon ultimately stemmed much of the tide that poured forth to annihilate Gondor. Maybe Turgon even built more vessels in plenty for in case of naval threats from the Corsairs of Umbar. Well, Turgon really seemed like a true leader of men in the fading of the Dunedain.