Rostock is near the Baltic Sea and to protect its fishing and access rights it actually annexed Warnemünde, a port area to the north.
The Airport Rostock-Laage  (RLG) lies near Rostock.
From Hamburg Central Station you can take the Regional-Express train to Rostock. Buy a Mecklenburg-Vorpommern-Ticket. With this MV-Ticket you can travel with 5 Persons from Hamburg to Rostock. Prise not IC or ICE.
From Hamburg take the motorway A1 to Lübeck and from there take the A20 to Rostock. It's a 1 1/2 hour trip. If starting in Berlin you have to drive along the A24 in the direction of Hamburg until the A19 is crossing the motorway. Following the A19 you reach Rostock. It's a trip of 2 1/2 hours.
Berlin Linien Bus serve Berlin to Copenhagen via Rostock 
From the ferry terminal, you need to take a series a buses to go to the center of Rostock. You can also try to head for the Rostock-North S-Bahn (train) station, about 1km away from the terminal. Therefore you may get stuck at the ferry port certain days when the bus services are low or if you can't find the train station. A taxi ride downtown will cost you around 20€.
The port of Warnemünde just north of Rostock is used by cruise ships, which dock at Hohe Düne. It can be reached easily by train (10 minutes) or taxi from central Rostock. All trains from Warnemünde stop in Rostock. The train station is an easy three minute walk north along the dock.
Rostock features a good inner-city tramway system. A bus system serves other locations. 
Café Lom, Barnstorfer Weg 19: studenty place with Indian-style food. Cheap and good. Café Central, http://www.cafes-in-rostock.de/central/index.php In the middle of KTV, just round the corner of Doberaner Platz. Popular with students and other guests. Breakfast from 9 to 14; afterwards (and from about 11am), food is served until 22:30. Mambo No 1 http://www.pub-rostock.de/index.php/Kontakt.html Exactly why an Irish pub would be called "Mambo" remains a mystery, but the place is comfy and features regular live acts. Drinking guiness is fine but the cocktail menu deserves a closer look, too. Likörfabrik, corner of Große Wasserstraße/Grubenstraße: nice little café, menu includes wraps as well as cakes. If you play the piano (read: play it well! It's students of the nearby HMT, the university of music and theatre, who are setting the standard), you can ask whether you may play for your drinks. Large groups may find it difficult to squeeze in when it's busy. Marientreff, Bei der Marienkirche 1a: Enter the cute little courtyard just behind Marienkirche. In late spring/early summer, you can sit beneath a blooming lilac bush. Secret spot for home-made cakes and hot & cold drinks. Run entirely by volunteers of the Marienkirche parish and open daily, 11:30 - 17:30. From time to time there are also readings or small exhibitions here. Schallmauer, Doberaner Str. http://schallmauer-rostock.com/ Cheap beer and hard alcohol, some food which is a selection of "Plan B"'s menu (restaurant across the road, probably same owner(?)). Suppenwerk, Doberaner Str. 111. Soups! Good, satisfying, budget-friendly. Take-away also possible. Weekly menu. Opening times Mo - Fr: 11 - 16. http://suppenwerk-rostock.de/
Jellyfish Hostel  It is situated in a beautiful old Town House in the Old Town of Rostock just around the corner from the medieval Steintor. It offers beds in dormitories (drom 17€ in summer) and privates (from 23€ in summer). fee of 2,50€ for bed linen. Though the hostel is in the very city center - the Neuer Markt is two minutes by foot - it is really quiet there. It has a very nice common room, where breakfast (4€) is served as well, with two free internet PC's, a flatscreen, a stereo and a projector. In the fully equipped kitchen you can cook or just store your beverages in the fridge. The 8-bed dormitories are very spacious due two the very high ceilings, which even have stucco. The owners and all employees are quite young, easy going and very helpful.
Hanse Hostel  - Doberaner Str. 96. Prices start at €14 for a bed in an 8 bed dormitory, for bed linen they charge €2 once per stay. Breakfast is available for €4, coffee, beer and soft drinks are available for €1. There is free internet, a great kitchen with a dishwasher, microwave, oven, stove and fridge. Bathroom facilities are very good. There is a TV, DVD and VCR downstairs as well as a bunch of board games, books and a dart board. The staff are very friendly and helpful and the place is clean, tidy and modern. Hanse Hostel is located at 96 Doberaner Straße near the corner of Lubeckerstraße, if your travelling by tram get off at the Maßmannstraße stop.
Jugendgästeschiff  - According to the Tourist Information Office in Rostock this is the cheapest place in town with prices starting at €30 including breakfast, €20 if you have a discount card. Jugendgästeschiff is on a ship in the harbour basically at the end of Am Kabutzenhof.