There are no ferries that go from Venice to anywhere near Split.
There was one through 2006 run by Eranmar (spelling?) I think it was. Sadly, it looks like it's been canceled. Sorry.
The overnight train (#421) from Venice to Zagreb has a good connection to the train to Split although it is very early in the morning.
That trip is also 15-16 hours, which is why I recommended car...
That train leaves Venice at 21:20 and I can't imagine passengers being off boarded in Trieste and bused to another train at midnight.
Please reread my original post. As I stated there are two options, "either go all the way up around Slovenia to Villach Austria (usually), OR go thru Trieste, where you have to disembark," You don't do this on the EN 241, as it routes completely up and around Slovenia to Villach, Austria.
My point being, both train choices are inconvenient: A, the "Trieste Shuffle", or B, a 15-16 hour train trip.
And yes, if you chose a route that went through Treiste night-train or not, you have to disembark (though this is not the case with the 241 as it routes around). The tracks through Trieste were blown in WWII and never rebuilt, due to the Cold War. Hence, all the bus/train recommendations you'll find on other sites.
If you're happy with what you've found, go for it! :)
If the RailEurope site it's lower, that would be a first! :-D