When you control no Islands, sacrifice Seasinger.
You may choose not to untap Seasinger during your untap step.
: Gain control of target creature whose controller controls an Island for as long as you control Seasinger and Seasinger remains tapped.
10/4/2004 You do not lose control of the stolen creature if it stops being a creature. You only lose control if Seasinger leaves your control or becomes untapped.
9/16/2007 If a creature’s controller doesn’t control an Island, that creature is an illegal target. Whether that player controls an Island is checked both when Seasinger’s ability is activated and when it resolves. However, targeting restrictions aren’t checked continually while a spell or ability is on the stack.
9/16/2007 Whether you control Seasinger is checked continually, starting when the ability is activated. If, before the ability resolves, there’s any point at which you don’t control Seasinger, the ability has no effect — even if you control Seasinger again by the time the ability resolves. The same is true regarding whether Seasinger remains tapped.