From central and southern Italy, there are year-round car ferries from Split to Ancona.

In July and August there are passenger ferries from Pesaro and Cesenatico to Rovinj, Mali Losinj, Pula, Rab and Novalja. See the schedule and book here.

From Venice to Croatia the catamaran routes are

three sailings a week to Rovinj beginning April 29, four weekly in June, daily July and August, five weekly in September, ending October 1;
three sailings a week to Porec beginning April 29, four weekly from mid-May to mid-June, five weekly from mid-June through September;
a weekly sailing to Pula in June, twice weekly through mid-July, three times weekly through mid-August, twice weekly and then weekly until September 19.

and Porec at the beginning of May and end at the beginning of October. Sailings are more frequent in July and August.

Operated by Venezia Lines, note that the schedules are arranged to make day trips from Croatia to Venice, not vice-versa. Atlas-Kompas provides additional sailings in summer to the same destination. No matter when you go, I strongly advise that you buy your tickets online in advance.