I want to travel from Venice to Rijeka, Croatia by ferry. We will be there in July. Is this possible, or do we need to stop at Pula and take a bus? Is there a website that can help me?
The best choice (if you want to travel only by boat) would be to travel by ferry to Rabac from Venice, then rent a "taxiboat" to Rijeka. Otherwise you need to travel to Pula and find something like that there. Moneywise I would say that the least expensive way to get to Rijeka is to take the ferry to Pula and then a bus from there.
Mine are just speculations... Normally a ferry between 2 big cities (Venice and Pula) should be cheaper (and more frequent) than a ferry between a big city and a smaller one. Also consider that Rijeka is farther than Pula, so I'm betting it costs more.
Also consider that bus lines have scheduled trips and can count on a plethora of customers (cheaper), but a taxiboat works "on demand" with only a few passengers (expensive)...
I cannot give a quote on prices, since they depend on the lines you are using, the days you are travelling and so on.
Rome2Rio suggests taking the ferry to Pula, and from there a bus to Rijeka. Below is a screenshot from Rome2Rio:
The buses are operated by Autotrans. They run approximately every 2-3 hours, and takes 2h20m to get to Rijeka. At the time of writing (Feb. 2016) the price for a one-way ticket is 86 HRK. Below is a sample timetable for buses leaving on the 13/07/2016: