My question is about rule 6.21. There is stated "only warships already occupying a lake may take part in an amphibious move" No.
The question is: you can move a ship to a lake in the naval phase and do a amphibious move in the same turn?
Or has the ship already be in the lake (moved in a previous turn). Yes.
If the answer is "the previous turn", then I don't get rule 6.2. This rule says: "Players may combine navy and army phases to make one amphibious move" WHEN you satisfy the 'conditions' to PERFORM this, THEN, it were allowed. It means you have to have had a 'Vessel' already located where this applied. The "Naval Move" OUT upon the 'Lake', has that 'performed' its 'allocation' at the time.
PLAN WELL ENOUGH AHEAD