Please see
@Rulebook:
…
Attacking or defending submarines that choose to submerge are immediately removed from the battle strip and returned to the
game board in the contested sea zone, removing them from the remaining battle sequences.
Note: Decisions on whether attacking and defending submarines will fire or submerge must be made before any dice are rolled
by either side. The attacking player decides first.
…
and
@Rulebook:
…
Once all attacking and defending submarines that conducted a Surprise Strike have fired, the casualties they have generated
are removed from the game and this step is over for this round of combat. As long as there are attacking and/or defending
submarines and no opposing destroyers, this step is repeated during each round of combat. Any hits made during this step that
do not destroy units (such as battleships) remain in effect until the end of the combat.
…
Please ask if anything remains unclear.
@Swift:
So a sub can suprise strike, submerge, then other sea units fire, then defending units fire (excluding submarines), then remove casualties, then press attack, then same sub suprise strikes and submerges, then attacking sea units fire… and so on until defeat or retreat of attacker?
No, a sub can either surprise strike or submerge. If it submerges it will be out for the rest of the battle.
If it decides to surprise strike in the first round of combat and survives this round of combat, it can decide
to submerge or to attack again in the next round of combat.
HTH :-)