2x crates require two (2) bubbles to stop, but give you double the points.
3x crates require three (3) bubbles to stop, but give you triple the points.
Bubbling pallets with cannons and stacks of bubbles on them will increase the number of cannons you have and their fire rate respectively. Make sure to bubble those as soon as you see them.
Bubbling the pallets with ice blocks on them will freeze all crates and pallets on the screen. Use these to clear as many crates as possible to create a little bit of breathing room during the level.
You can bounce bubbles off of the ceiling. This can be used to bubble crates or rescue downed survivors.
If a survivor is bonked and trapped in the sand, use a bubble to pull them out and get them back in the game.
If you collect the extra cannons, be sure to protect them from the falling crates. If they get hit, they will be destroyed.