I’ve been a fan of Sealab 2021 for quite a while now, along with other Adult Swim shows which have the same stylistic elements, such as Space Ghost Coast to Coast and Harvey Birdman. It looks like the animators use motion tweens to control the characters as if they were puppets, but these days the bone tool would be used to more efficiently do the same job.
Sealab is great a satire of old Hanna-Barbera cartoons like Johnny Quest, which is where it derives much of its style from. Although there isn’t a wide range of movement within the characters’ bodies and facial movement like you would see in old episodes of Johnny Quest, the animation is still effective delivering the humor. Sealab doesn’t need a lot of action animations to move the story because it really isn’t for kids, but the humor is still there.