The waterline can present a problem to vessel owners dealing with tight budgets. They don't want to flood their ships, obviously, but spending extra money on technology that isn't needed isn't the best course, either. For new ship owners facing renovations, doors at the waterline pose a problem because the wrong type of door can lead to flooding in the ship during rough weather. Being able to choose the right door can be the difference between a ship that stays afloat and one that quickly takes on water, which will only cost the owner money.
How Much Time Does That Part of the Ship Stay Under the Water Line?
The biggest issue is how much time that part of the ship may be underwater. Something right at the waterline that is frequently submerged by wakes and wave action may be better off with watertight qualities instead of weathertight (weathertight doors are designed to withstand only occasional blasts of water). A door that is just above the waterline that really doesn't see much aquatic action could be weathertight instead.
How Vital Is It That the Two Connected Compartments Be Open to Each Other?
If the door is to go between two compartments inside the ship and the compartments are below the waterline, then you have to make the door watertight. But then you have the choice between different types of watertight doors. Some are meant to stay open most of the time, while others are meant to be closed most of the time. These doors are often designated by types with a specific letter. The choice is similar to choosing emergency stairwell doors where you have fire doors that need to stay closed versus plain doors that can stay open.
How Often Do People Cross That Threshold?
Some watertight doors are powered, meaning you don't have to open or close them manually. The problem with powered doors is they can close at the command of someone in a different room, usually a control room, instead of being powered right at the door. This setup allows a ship's crew to close all doors as soon as possible, rather than having to run all over the ship to ensure the doors are closed in an emergency. That means people crossing the threshold between compartments are at risk of injury if they don't pay attention to signals that the closing mode has been activated.
There are additional choices that you can make regarding watertight doors you add to a ship during renovations. However, power, the waterline, and whether the door needs to be mostly open or mostly closed are among the biggest issues.