Any block the player can walk through, regardless of whether you can see through it, the guardians can shoot through it anyway. This is quite glitchy because it means that guardians can shoot right through seaweed type blocks. I think this could be fixed by making guardian lasers become obstructed by block hitboxes instead of collision boxes.

This can be confirmed for: powder snow, lava, open fence gates, cobwebs, kelp, seagrass, flowers, tallgrass, ferns, coral "plants", coral fans, rails, sugar cane, and vines.