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

The twist-on Easy Lock sealing ring mates with your boat's Easy Lock 20A or 30A Easy Lock shore power inlet.
The Easy Lock ring will not, however, mate with older style threaded inlets. If your boat has a threaded inlet, purchase the 103RN version of this cover with a threaded sealing ring, instead.

Note that if you have an Easy Lock Inlet on your boat, it will have provisions to accept either Easy Lock or Threaded styles of sealing rings on your power cord connector.

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 Easy Lock ring removed, these covers can also be used on 20A and 30A male plugs, as they are the proper diameter and length.