Hm, if the USS Texas goes down, would it be possible for the US to warn that any submarine spotted will be considered hostile unless they surrender to an inspection by USN/USCG vessels? After a couple more submarines are found and destroyed, Hitler is incensed enough to declare war.
11 September 41 Roosevelt took a step further than that with the 'Shoot on Sight order:
http://ww2cartoons.org/sept-1941-roosevelt-orders-navy-to-shoot-on-sight/
I've not been able to confirm, but its claimed this was one of the several incidents when Hitler discussed a DoW on the US.