It is not security but securite pronounced as archduke said. Here is the info from the U.S. Coast Guard on the proper procedure to transmit:
Transmission of the safety message:
tune the transmitter to the frequency or channel indicated in the DSC safety call;
transmit the safety message as follows:
"SECURITE", repeated 3 times,
"ALL STATIONS" or called station, repeated 3 times,
"this is",
the 9-digit identity and the call sign or other identification of own ship,
the text of the safety message.