What causes this deck edge looking weird at the plan view. There's an error that occurs when doing floodable length analysis.
First, turn on the Margin Line icon to check if there is a problem with the defined Margin Line of the hull. The weird spikes is the deck line not snapping to the hull properly. It tries to automatically find the deck edge by looking for the sudden change of shape in cross section, sometimes it fails. What you can do is to manually edit it, go to Input Window | Margin Line Points. Lower the values of the height where there are sudden changes of shape. Then after editing a value, go to Analysis | Snap Margin Line to Hull.
Once this has been done for all the points that need to be changed, selecting Snap Margin Line to Hull in the Analysis menu will project all of the points horizontally onto the hull surface, ensuring that the margin line follows the hull shape precisely.
The original points have been specified on the right side of the table to show the changes in height. Please refer to the Stability Manual - Margin Line points pages 110-111 for more information.
You may also check this wiki for reference: