3.b. Dragon's CIP ability resolves, removing your opponent's lands, Jet, and Animate. Animate Dead's leaves-play ability goes on the stack.
3.b.i. You activate Seasinger, and gain control of Dragon. Go to 4.
You can't even activate Seasinger with the leaves play ability on the stack, your opponent doesn't control an island.
Seasinger
1UU
Creature — Merfolk
0/1
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[/B] as long as you control Seasinger and as long as Seasinger remains tapped.