[grobdriver]

What does this mean to the ships at sea?

Not much.
Watches still have to be stood. The ship remains on station.

This will be a day where all departments review their proceedures for safety.

We would do this in the VP Navy whenever there was an incident or accident.

[encm(ss)]

You are exactly correct. Submarines at sea stay at sea. However the next return to homeport will have the Squadrons perform inspections to ensure that SOP's bills are written and are being complied with as they are written. Generally speaking - administrative check-up and enforcement.
Retired MCPOC(SS)

[Doohickey]

Pretty much. This sort of thing happened once when we were deployed to the Med. We did training instead of drills for three days on the morning watch, and had to spend a day watching more training when we arrived in La Maddalena for a scheduled upkeep.

[Kozak]

We would do this in the VP Navy whenever there was an incident or accident.

USAF would do this periodically too when ever there is a rash of accidents.