Succinct… you be the judge.
But enemy subs do not block sea movement.
So any unit, including transports, can ignore them during combat move phase. However, if you ambitious assault from a SZ with enemy sub(s) and the transport is unescorted, your opponent can defend with the sub, which would stop the assault.
If transport has at least 1 warship (aircraft do not count since without DD they can’t hit) with it, the attacker has the option of ignoring the sub, which then allows the assault to commence.
If the attacker wants to fight the sub, a naval battle occurs and if the attacker wins, the assault proceeds. Note: in this scenario, the enemy sub has the option to submerge, if no destroyers are present.
Here’s a cut and paste from the rule book:
Enemy submarines and/or transports don’t block any of your units’ movement, nor do they prevent loading or offloading in that sea zone (with one exception; see ‘Special Combat Movement: Transports‘). As the moving player, you have the option of attacking any enemy submarines and/or transports that share a sea zone with you. However, if a unit chooses to make such an attack, that unit must end its movement in that sea zone, and it must attack all such units present. In other words, you must either attack all enemy submarines and transports in the sea zone, or you must ignore all of them. You may not attack some enemy units and ignore others in the same sea zone. It is possible that some of your units stop to attack while others continue moving through the sea zone.
Special movement section
A transport can load units while in any friendly sea zone along its route, including the sea zone it started in. If a transport loads land units during the Combat Move phase, it must offload those units to attack a hostile territory as part of an amphibious assault during the Conduct Combat phase, or it must retreat during the sea combat step of the amphibious assault sequence while attempting to do so. A transport that is part of an amphibious assault must end its movement in a friendly sea zone (or one that could become friendly as result of sea combat) from which it can conduct the assault. However, a transport is not allowed to offload land units for an amphibious assault in a sea zone containing 1 or more ignored enemy submarines unless at least 1 warship belonging to the attacking power is also present in the sea zone at the end of the Combat Move phase.
