When Cassim discards the Hand of Midas by throwing it into the ocean, it accidentally hits the ship that was used by the remaining seven thieves who evaded capture by the Agrabah palace guards and were tricked by Saluk that Cassim sold the thieves out. The ship turns gold and sinks, and because the Vanishing Isle had submerged and would not rise until long after and there's no other land nearby, they must have drowned. And worse Cassim did NOT seek any vengeance on the Thieves for their betrayal.
Cassim turning the Vanishing Isle temple into gold probably also drastically increased its density and weight. That turtle may never surface again.
Not so much on two accounts: One, the hand only turned the inside of the temple gold, not the outside as suggested above. Two, the turtle did resurface if only to let Genie out as Steamboat Willie.
Plus, the turtle is a giant and can resurface despite having a large temple on its shell. So a few more density and weight won't make a difference.