Don't be scared.
They have motion stabilizers that keep the rocking to a minimum. April is a great time for a cruise because the weather is usually good. You'll feel rocking the first and the last night, because they tend to move faster to reach their destination on time.
There's tons of stuff to do in Aloft/The Stack. The counsellors are great, the games and activities are great and if you don't really feel like you fit in there, there are always activities going on around the boat, there's always a pool to swim in, always movies to see, etc.
Besides. It's not a normal boat. It's like a mini resort city that moves from place to place.