Blue Robotics released their Pressure Relief Valve (PRV) in 2022 but we still get questions about the difference between it and the Enclosure Vent and Plug.
The PRV has the functionality of an enclosure vent in that it can be opened manually and used to perform a vacuum test on your enclosure, but it also automatically releases any excess pressure built up inside a watertight enclosure. This automatic venting feature is very useful when you work with the BlueROV2 in warm climates or the enclosures are in direct sunlight while you prepare to launch.
The PRV plug (blue), when used with the backfill adapter (red), allows you to fill an enclosure with an inert gas or to pull an enclosure to vacuum with a vacuum plug (brass) while maintaining a seal.

So, while the PRV is a little more more expensive, the added functionality and versatility is well worth it.
And we recommend you keep a spare plug handy for both the vent and PRV, in case you lose one in the field.