Ferry from Venice to Croatia, May, 2018

Where in Croatia? It has a very log coast.
If you put the name of your town in the search, you might get better results.
I tried "Ferry Venice Pula" and got this: http://www.croatiaferries.com/venice-pula-ferry.htm
"Ferry Venice Rijeka" got nothing - I don't think there is such a route.
"Ferry Venice Zadar" and got a ferry from Ancona (further down the coast from Venice) to Zadar: https://www.directferries.co.uk/zadar_ferry.htm

Yes, it would help to know where you wish to go in Croatia.

If you are talking about the passenger ferry/catamaran from Venice to Istria, this might be it:

http://venezialines.com/

But no 2018 schedule there yet, either. Personally, I wouldn't worry about it yet - keep checking (or try to find out what the May 2017 schedule was and guess it might be about the same). Meanwhile, plan an alternate means of transportation. For example, the Slovenian transfer company GoOpti provides shuttle service from Venice to Istria; you can at least see what prices might be (prices will go up as you approach departure date).

Taking the boat from Venice to Istria is usually available 4 days of weeks in May and you are spending the time inside. Journey can take from 4 to 5.5 hours depending on your arrival town and is expensive way to travel compared to bus or train/bus combination. Another disadvantage with the boat is your departure time around 5pm.

Last year's schedule for Venezia Lines is at http://venezialines.com/wp-content/uploads/2017/01/VL-Schedule-2017-Draft-1.pdf. I would not necessarily look for 2018 schedules until about February. But Adriatic Lines seems to be booking already for May, if you like their schedule: http://adriatic-lines.com/en/time-schedule-to-venice/.