Per all published sets of revised rules, tanks defend on 3.
As for spending money on German navy…
Leaving the Batlic at starting levels means it is sunk in UK1, period.
Adding an AC, even if you do not put FIGs on it, means that fleet will be around into turns 3, 4, or 5.
A German navy EXISTING means that the UK and US can;t just build TRNs, they HAVE to build capital ships to defend those TRN’s.
That slows down the Allies. Even a single turn where Germany faces ONLY Russia is a massive advantage, both in tersm of force superiority AND in terms of economic increase.
It costs 16 IPC’s to keep the Baltic fleet alive a MINIMUM of another 3 TURNS. 16 IPC’s is 5 INF and an IPC… and those 5 INF are wiped out by the first Allied invasion in Turn 3 if the US and UK ONLY have to build TRNs and units to ship, and not worry about protecting them.