Flooding has a total of 40 possible levels. At various levels, functionality is lost, and drowning is definitely a major concern when the water level goes exceedingly high.
Level 1: Water can be collected anywhere.
Level 3: Mining is unsafe.
Level 4: You may not access your chests safely.
Level 5: Nurturing seedlings is unsafe.
Level 10: Altars cannot be accessed.
Level 15: Some sort of floation is required in order to not drown.
Level 18: Hunting now results in fish and other aquatic animals.
Level 20: Trees are now submerged and cannot be accessed.
Level 25: One must use specialized diving equipment to dive for scrap piles.
Level 35: Manmade structures and buildings are destroyed.
Level 40: The flood begins to rapidly dissipate if it has not already.