The destructive obstacles include water, fire and bombs, named because they are the only obstacles which can destroy objects. Only non-moving monsters can technically be classified as destructive obstacles, because moving monsters are classified as objects.
Bombs destroy all objects; water does not destroy Chip with flippers on or gliders. Fire is more complicated, as it rejects the walker and bug and allows the fireball through, and will not destroy blocks or Chip with fire boots on.
Destructive obstacles are immovable; fire is indestructible, water can only be removed by blocks, and bombs can be removed by blocks or monsters.
Strategies for removing blocs of bombs or water
When confronted with bombs and a choice between blocks or monsters, note that there may not be sufficient blocks and that redirected monsters may be required to pass the requisite bombs. Also note that there may be water in the bomb stream, such that only the blocks will suffice. If the monsters are gliders, remember that you can send them over the water before you remove the water; using any other monsters, Chip would have to remove the water before any more bombs could be removed.