That submarine is making a battle with an average +10 value swing there (140% the value of the attacking force!), which is more than sending it to kill the lone British transport
Actually, sending the German submarine which is in the North Atlantic to kill the transport next to Eastern Canada (average +7) instead of taking the shot against the 1 cruiser and 2 transports next to Eastern United States (average +10) is some less than a -3 difference since, if using it to kill the British transport, then nothing can kill the submarine on the same round, whereas the 50% chance of the submarine surviving next to Eastern United States implies the ability for the Americans to make a battle with 1 destroyer, 1 fighter and 1 bomber (or even more units) against 1 submarine, which is an about +4.4 value swing, to be reduced to +2.2 on the account that the submarine will be there only 50% of the times. So that covers at least most (2.2/3) of the difference between the two options the submarine has if you actually think that having that submarine alive next to Eastern Canada at the start of the second German turn is actually worth its value or more. Opinions?
In any case, the battle next to Eastern United States is certainly a good option which your opponent may take and, in this case, it will be a 50/50 on either:Germany having 1 less submarine for nothing. America having 1 less cruiser, 2 less transports and having to invest units to kill 1 German submarine, with slightly more than a 1/6 chance also to lose 1 destroyer in the process.