No there is a need to roll dice, as subs are ignored and do not result in combat on their enemies turn unless the enemy ship is an unescorted transport. Then the sub gets one shot to try to sink it outside of normal combat, this shot occurs as the transport moves in or through the zone. It is listed separately in the rules and I did not know about it for the longest time in pacific until I read the sub section in the rulebook. Attacking transports are never defenseless as they can always retreat. However this is not an attack, this shot occurs during movement of any type.
See page 30 AAE40 submarine section under: “Doesn’t Block Enemy Movement” line 9 beginning with “There is an exception…”