Rulebook P2E, Page 13 (Chapter CM): If you are sharing a sea zone with surface warships (not submarines and/or transports) belonging to a power with which you are at war, this situation requires you to do one of the following:
• Remain in the sea zone and conduct combat,
• Leave the sea zone, load units if desired, and conduct combat elsewhere,
• Leave the sea zone, load units, and return to the same sea zone to conduct combat (you can’t load units while in a hostile
sea zone), or
• Leave the sea zone and conduct no combat.
Once these sea units have moved and/or participated in combat, they can’t move or participate in the Noncombat Move phase of the turn.
According to the rules, 1-3 is combat move+combat, but 4 would be a non combat move.
Not sure if it would have been emphasised that the leaving takes place exceptionally in CM phase. Also the last sentence would be clearer if it stated “In all of these cases, units can’t move or participate in the Noncombat Move phase of the turn.”
Edit: I have to apologize, this exeption is actually printed on page 12:
However, units can’t end their movement in friendly spaces during the Combat Move phase except in four instances:
(…)
• Units moving from a hostile sea zone to escape combat as their combat move.
A sea zone into which defending air units may be scrambled in reaction to an amphibious assault (see “Scramble,” page 15) may be treated in the same way as a hostile sea zone for this purpose.
So scrambling fighters also block NCM this way.
Page 22 (Chapter NCM):
Transports can move to friendly coastal territories and load or offload cargo, unless they loaded, moved, offloaded, or were involved in combat during the Combat Move or Conduct Combat phase.
So this means having an enemy vessel in the seazone at the beginning of the turn does automatically mean battle activity for every naval unit in that SZ, including SSs.