Rental Car options in Warnemünde port?

Check the berth of your ship: https://www.rostock-port.de/en/ship-traffic/cruise-shipping/arrivals

Only P7 and 8 are the cruise terminal.

Is there a reason why you do not want to use trains or Flixbus?

Driving into Berlin with a car you do not know can be stressful plus administrative things such as driving permission for Germany - not license only.

Never rented there. Looks like a small container office with brands mentioned.

The Autobahn between Rostock (Warnemünde) and Berlin is great, and I enjoy using it. However, if you don't want to do a lot of sightseeing along the way, I would definitely take the train. But if you want to stop on the way to Berlin, we for sure would have some great tips. By the way Warnemünde itself is worth spending a few hours there and Rostock is also worth a visit.

https://www.europcar.com/en-us/places/car-rental-germany/rostock-warnemuende/rostock-cruise-terminal-warnemuende
On the map, you can see that Europcar is located almost next to the Warnemünde S-Bahn station - you could take the S-Bahn to Rostock Central Station, and from there, take the train to Berlin.

Thanks. We are going into berth 8. My only hesitation with train is concern about train strike although maybe the frequency we hear about is overblown? We do have IDP already and lots of experience driving in foreign countries (including Japan - husband is a retired marine and we’ve lived all over but not in Germany). I will definitely price out train options (there are 4 of us traveling.). Any tips on train travel? We are used to Shinkansen and local trains in Japan.

I would not worry about a train strike, and the train seems a lot easier. Driving to Berlin, and especially driving in Berlin, sounds like a nightmare.

In the peak and on summer weekends the S Bahn runs from Warnemunde to Rostock every 7 1/2 minutes, and a broadly hourly IC or ICE train from Rostock to Berlin.
Most of the day the S Bahn is every 15 minutes from memory - long double deck trains
I don't know if they still run but there were a few through ICE trains to and from Warnemunde - I know as I've been on one.
They may now either be dated services or withdrawn as being superfluous to needs.

So check train options between Warnemünde and Berlin Hbf (Central): https://int.bahn.de/en

The journey planner includes planned maintenance impacts which happen until end of November: no regional and no long-distance train is getting to Warnemünde. All travelers need to change at Rostock Hbf.

In summer driving A 19 can be a bit painful because a lot of vacation and weekend travelers to / from shores. The part Wittenburg and Berlin Autobahn ring is known for traffic jams.

Berlin has severe issues on the city Autobahn.

Thanks for all the suggestions and advice! I did book train travel. Will leave a review after we do the trip in early July.