From the rulebook about submarines:
Does Not Block Enemy Movement: The “stealth” ability of submarines also allows enemy ships to ignore their presence.
Any sea zone that contains only enemy submarines does not stop the movement of a sea unit. Sea units ending their combat
movement in a sea zone containing only enemy submarines may choose to attack them or not. Sea units can also end their
noncombat movement in a sea zone containing only enemy submarines.
About transports
Any sea zone that contains only enemy transports doesn’t stop the movement of a sea
unit. Air or sea units (other than transports) ending their combat movement in a sea zone containing only enemy transports
automatically destroy those transports (unless they are ignoring them to support an amphibious assault instead). This counts
as a sea combat for those sea units. Sea units can also end their noncombat movement in a sea zone containing only enemy
transports.
If I interpreted this correctly, a seazone containing both submarines AND transports blocks the movement of sea units. This doesn´t make any sense.
Do submarines have the ability to protect allied transports? (obviously not from air units) Can you ignore making an attack on a seazone containing transports and submarines to make a seashore bombardment?