We went to elephant Island last year, it is well worth seeing, I dont know if you are planning to do the trip from Goa, but we actually were staying in Mumbai when we took the trip, the boat trip is really interesting, as it goes past a lot of military forts that are in the sea, when you get to the island , as some one has already said avoid the so called tout guides, they are a complete rip off, we got stuck with one, and could not get rid of him, the views and the ancient carvings are truly wonderous to see, it will always amaze me how these carving were done, and on Eelephant Island they have actually carved into the mountains, the statues are fantastic. The monkeys are also well worth seeing they are quite tame, and surround you as you walk up the mountain track.
A trip well worth doing, if you are staying in Mumbai it will cost you about 400rps each, thats with the boat trip over to the island, but I dont know how much to trip organisers cost.