As of 2018, the base fare for a one-way, 75 minute valid ticket is 7,50 Euro (for visitors; residents do receive reduced pricing). Yes, that's pretty high, and that's why it may be to your benefit to buy one of the multi-day travel cards currently available. Their prices (again as of 2018) breakdown as follows:
24 hours: 20 Euros
48 hours: 30 Euros
72 hours: 40 Euros
7 days: 60 Euros
See a complete listing of current vaporetto fares.
What's your best option? That depends on your plans and the length of your stay, obviously. If you are only in Venice for a few days or it's your first trip there, and you plan on staying primarily on the main island itself, you may do best only purchasing single-ride tickets as and if you need them. However if you plan on exploring other islands in the lagoon, or maybe you're staying in a hotel located farther away from the main attractions you want to see, you could easily benefit from one of the multi-day passes.
I'll give an example: On my most recent trip to Venice, we were staying there for 8 day. Three of those days would be busy with a group event where the organizers would be providing travel arrangements to and fro, so I knew we wouldn't need to use the vaporetto then. So we bought 3-day passes for the first few days of the trip and made good use of them, traveling to San Lazzaro degli Armeni one day which we had never visited, heading back to our apartment in Dorsoduro several times at night when we'd been exploring in Cannaregio, and a couple quick hops along the Grand Canal on the 1 line. All in all I found that we would have spent 105 Euros traveling by single passes versus the 80 Euros we spent buying two three day passes, so that worked out well. We also bought 24-hour passes for our last "day", to visit Torcello and Burano, and then to take the water bus to the train station the next morning before our passes expired. Again, three rides that would have cost us 45 Euros individually instead cost 40...a small savings but a savings nevertheless!