Navis with land blocks may move to sea as part of a group move.
Presumably, as a group move it can move twice if not attacking?
Right, all units (not just land units) can move 2 spaces if not attacking.
Amphibious movement can be used to sea retreat, but destination port must be friendly then it adds : “on that same sea only” 7.64
What does that mean? That all sea retreats for the whole battle may only use the same sea?
No; it just means that you cannot retreat a block more than one "hop" along a navis amphibious chain. A normal amphibious move can go from port to navis (in sea adjacent to that port) to navis (in another sea adjacent to that sea) and so on. An amphibiously retreating unit can only go though a sea to a port, not through an arbitrary number of seas.