It depends on what you mean by "easy." To me, ferry travel is pleasant, but whether it is easiest is dependent on your preferences and on how much luggage you are hauling around.

I have not take these specific ferry routes before, but I have traveled the Adriatic by ferry. You have to expect to be delayed occasionally by bad weather, but in general the ferries seem to run close to schedule. Slovenia has a very small coastline, however, and Piran or Koper would be just about your only choice to land literally in Slovenia. That trip would take around 7 hours from Venice. Then going by ferry from Slovenia proper to Croatia's coast could involve ferry trips anywhere from a hour to many hours or even overnight, depending on your planned destination in Croatia. The generality of your question doesn't allow me to make much more specific observations.

Since you seem to traveling totally by public transport, I'd advise you to travel by bus or train from Venice into Slovenia. From there into Croatia, assuming you are most likely in an inland Slovenian location, bus or train will probably be your best choices.

If you want to ride a ferry just for the experience, get to the Croatian coast first. Then take a short ferry ride to one of the islands. Good luck.

There aren't really any ferries from Venezia to Slovenia or Croatia. There are several fast craft which offer day excursions from Slovenia or Croatia to Venezia; you can buy a ticket to travel from Italy on these but they go from Italy in the evening.
It is much easier to get a train from Venezia to Trieste (hourly trains taking about two hours) then a bus. I've travelled this way to Koper, Opatija and Rijeka, but the best route for you depends on where you're going.

I traveled by bus from Trieste (2 hours by train from Venice) to Rovinj, which is on the Istrian Peninsula. You travel through the brief coastal parts of Slovenia.

Your trip planning will depend a lot on where you want to go in Croatia. Train travel is sparse but bus and boats are plentiful.

My route was bus to Rovinj (4 days), then bus to Rejika then ferry to Dubrovnik (4 days) then fly to Zagreb and train to Lake Bled. You can get some very cheap flights from Dubrovnik to Zagreb and save some time.