As noted above you report it on your sea service
Getting your time into a logbook is the way to record
it. You should sign and date your logbook entries as well.
If you crew on others boats have them sign your logbook too where you can.
You want to establish a consistent record
of your activities. This gives weight to your records should there be any question. No blank pages, just a series of log entries that document your activities. It becomes a legal
record (but I'm not a lawyer!)