1 Erwin: Take this truck und drive north.
2 Erwin: Take these papers. They will get you past any Nazi checkpoints.
2 Monty: What do they say?
3 Erwin: You are collecting water from die Baltic to top up die Führer's jar.
4 Prof. Jones: Sea water? Won't any guards think that's a bit fishy?

A somewhat common factoid perpetuated by various Internet factoid lists is that human blood has the same salinity as sea water. This is poppycock.

Sea water has a salinity ranging between roughly 3% and 4% (by mass), depending on where in the world's oceans you take your sample. Human blood - and in fact that of almost all vertebrates - is pegged very close to 0.9% salinity, which is a hugely significant difference. It's the reason, for example, why drinking sea water is an incredibly bad and dangerous idea. All that excess salt has to be excreted by your kidneys to maintain your internal chemical balance, and the only way to do that is to pump it out in your urine in a more dilute water solution matching more closely your body's salinity. The result is that you get critically dehydrated very quickly.

(This is not to say that the small amounts of sea water you will inevitably ingest when swimming are dangerous. Just have a big glass of fresh water afterwards. You'll probably need one to wash the salty taste out anyway - I know I always do.)

Presumably if the Führer's brain really is kept in sea water, it's mixed with enough fresh water to reduce the salinity to a more reasonable level.

Or maybe Erwin is just making stuff up.

Or maybe this is a comic and none of it's real anyway.

EDIT: As it happens, I overlooked an important fact about the Baltic Sea. As reader Mikko Rasa writes:

I'd like to point out that the Baltic Sea actually does have a suitable level of salinity. To quote Wikipedia:

"The open surface waters of the central basin have salinity of 0.6 to 0.8 %. At the semi-enclosed bays with major freshwater inflows, such as head of Finnish Gulf with Neva mouth and head of Bothnian gulf with close mouths of Lule, Tornio and Kemi, the salinity is considerably lower. Below 40 to 70 m, the salinity is between 1.0 and 1.5 % in the open Baltic Sea, and more than this near Danish Straits."

So the surface water actually has a lower salinity than human blood. That's not to say it's necessarily very good for use in a brain jar, with all the micro-organisms and such. I don't know if the comic characters are aware of this, but you neglected to mention it in the comments so I thought I'd notify you of it.

Why do I know this? I happen to live in Southern Finland and do scuba diving as a hobby, so I've picked up a factoid or two about the local oceanography.