“An enemy submarine and/or transport does not block any of your units’ movement, nor does it prevent loading or offloading in that seazone.” (AA42.1, Rulebook, p.11)
“An enemy submarine and/or transport does not block any of your units’ movement, nor does it prevent loading or offloading in that seazone with one exception: a transport is not allowed to offload land units for an amphibious assault in a seazone containing on eor more ignored enemy submarines unless at least one warship belonging to the attacking power is also present in the seazone at the end of the combat move phase.” (AA42.2, Rulebook, p.11 and 13)
In my interpretation of the rules, the answer to your question would be: in AA42.1 you don’t prevent the offloading, in AA42.2 you do.