The rulebook is half clear. Â It states that all combat moves must end in combat on Page #13
Sea units that will be participating in an amphibious
assault from a friendly sea zone, as well as sea and/or
air units that may be needed to support it in the case that
defending air units are scrambled (see ï¿½Scramble,ï¿½ page16).
My interpretation of that is scramble or no scramble, or even no airbase, I can move whatever I want in the combat phase to a friendly seazone to support that invasion.
Your interpretation is incorrect. You may only move units that will participate in or support the amphibious assault into a friendly sea zone. Air and sea units that will participate in the assault include transports offloading land units, battleships and/or cruisers bombarding, and possibly a warship to escort the transports (if needed because of an enemy sub that will be ignored - bombarding battleships and/or cruisers can fulfill this role). Supporting units can include any type of air or sea unit desired, but can only be moved there if necessary due to the possibility of a defending scramble - there being both an operative enemy air base and eligible air units present.
As others have pointed out, the same criteria apply when determining whether units beginning the turn in the sea zone may move out of it during combat movement. Such units restricted from moving out in noncombat movement are those that were actually involved in combat and transports that offloaded.