A plug and socket type electrical connector for connection underwater. The female part of the connector contains a socket element enclosed in a dielectric fluid filled chamber which is sealed by a penetrable seal element. The plug or male part of the connector has an extended contact probe of round cross-section which penetrates the seal element to enter the socket and complete the connection, the two connector parts having housings with a threaded connection for securing the connector. The penetrable seal is specifically designed to accommodate repeated insertion and withdrawal of a male probe of round cross-section without loss of dielectric fluid or water leakage. The seal design does not restrict the male probe size, making the connector particularly adaptable for use with a probe having multiple electrical contacts arranged coaxially. It is further adaptable for use with coaxial cable having a central conductor and a concentric shield, the connector maintaining the integrity of the shielded cable.