How can I travel from Venice to Rijeka or Pula over land or sea?

Venice to Pula can be done in several ways:

Bus to/from Pula: http://www.pulainfo.hr/en/prijevoz/bus/54/ There are direct buses, and buses that only go as far as Trieste, but Venice - Trieste is easily done by train. In Trieste the bus station is next to the train station. Bus station website: http://www.autostazionetrieste.it/ There are also to Rijeka from there. Train Venice to Trieste: http://www.trenitalia.com

To Rijeka you can also go by train, but that involves getting from Gorizia Central to Nova Gorica. (best take a taxi). However, Nova Gorica is an intersting station, as the border between Italy and Slovenia runs right through the middle of the square in front of the terminal building. See http://www.bahn.de for schedules.

Another option is by ferry to some place on the Istrian coast, and then by bus. However most ferries from Venice to Istria are operated by tour operators offering day trips to Venice to tourists vacationing in Istria. They thus go to Venice in the morning, and return in the evening, often to late for onward travel. I travelled that route in 2003, and stayed onvernight in Rovinj before travelling onward to Pula. The ferry I had found by just walking up to the Stazione Marittime San Basileo and asking. (I did have the bus as plan B).

I second @geeo's proposal. Ferry is by far the most convenient option.

As an alternative, you can go by train from Venice to Trieste in roughly 2 hours. Trains run quite frequently. From Trieste you can take a bus to Rijeka. Another 2 hours and something. It's longer than the ferry but you won't get seasick. The total price will be at least 20 Euros.