Marinco's 20A/30A female connector cover comes in two versions.
This version (103RN) has a traditional Threaded sealing ring.
The other version (103ELN) comes with a twist-on Easy Lock sealing ring.

The threaded sealing ring mates with the threads on your boat's 20A or 30A shore power inlet.
If your boat has an Easy Lock shore power inlet, it has provisions to accept either Easy Lock or Threaded styles of sealing rings on your power cord connector so you can use either this connector cover, or the 103ELN version of this cover with an Easy Lock sealing ring, instead.

Use these covers with female connectors (on boat end of your shore power cordset or pigtail adapter) for a superior seal. Sometimes this cover is described as a "new" (the N in the part number), or as a "short" cover.

Fits:
20A connector 205CRCN
30A connector 305CRCN
Constructed of heavy-duty vinyl
Provides a weatherproof seal and a superior grip

Note - with the black Threaded ring removed, these covers can also be used on 20A and 30A male plugs, as they are the proper diameter and length.