You mean ANZAC and UK right?
Japan announces at start of it’s turn whether or not Japan will declare war on UK/ANZAC.
When this war is declared, USA is allowed to announce DoW on Japan at USA’s turn.
So basically, during the Japanese turn, Japan and USA are not at war, right?
So is Japan then allowed to float his boats through USA boats?
Correct, as long as Japan doesn’t actually attack the USA then it is not at war with USA.
And then it could indeed ignore US (sea) units.
Is this also the case in Alpha +.2 ?
On the turn that Japan DOW on ANZAC/UK it can still ignore USA fleet provided it doesn’t attack US/DOW on US
Yes, this is the case in Alpha +2.