The Seamoth Perimeter Defense System is a defense-oriented Upgrade Module for the Seamoth. The module is crafted in the Vehicle Upgrade Console and must be applied to the Seamoth's upgrade panel to function.
The Seamoth Perimeter Defense System is used by left clicking after selecting the icon with one of the hotkeys (1-4). The hotkey used depends on where the module is placed in the upgrade panel. If the left mouse button is clicked the Seamoth will generate a small charge that uses 1% of the Seamoth's energy. If the left mouse button is held a large radius blast can be produced that uses up to 30% of the Seamoth's energy.
Spoiler alert: The following section contains story related material.
When using Ion Power Cells, a simple blast uses 0.2% of energy, while a fully charged blast uses 6%.
It must be noted that killing aggressive fauna such as Stalkers and Sand Sharks is rather difficult without a large number of fully-charged blasts, which is highly costly on a Seamoth's energy reserves. Often, the better solution is to use a low-mid charge shock, which will stun predators and allow one with time to escape. Installing multiple Seamoth Perimeter Defense System allows multiple blasts to be fired off in quicker succession. However, oftentimes only one module is required for efficient defence.
The Seamoth Perimeter Defense System does not affect the Ampeel, presumably because of the Ampeel's electrical nature. The Seamoth Perimeter Defense System is capable of fending off Reaper Leviathans with only minimal damage to the hull. It is also a good defense against Warpers, as a charged shot is enough to cause them to temporarily warp away.