Seasinger

When you control no Islands, sacrifice this creature.
You may choose not to untap this creature during your untap step.
{T}: Gain control of target creature whose controller controls an Island for as long as you control this creature and this creature remains tapped.

Masters Edition

When you control no Islands, sacrifice Seasinger.
You may choose not to untap Seasinger during your untap step.
{T}: Gain control of target creature whose controller controls an Island as long as you control Seasinger and as long as Seasinger remains tapped.

Fifth Edition

If you control no islands, bury Seasinger.
You may choose not to untap Seasinger during your untap phase.
oc
T: Gain control of target creature whose controller controls any islands as long as you control Seasinger and Seasinger remains tapped.

Fallen Empires

Bury Seasinger if you control no islands.
oc
T: Gain control of a target creature if its controller controls at least one island. You lose control of target creature if Seasinger leaves play, if you lose control of Seasinger, or if Seasinger becomes untapped. You may choose not to untap Seasinger as normal during your untap phase.

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.

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.

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.