Day trip to Venice from Florence?

It is possible, but you won't get to do much. Once you get to Venice you will need to get a water taxi or vaporatto - depending on what you are going to do or see - and that all takes time. We did two nights and still did not scratch the surface. If you are going to all that trouble - it would be nice to do at least one night. Sometimes people try to get too much done and wind up not really enjoying the trip because you are so rushed. It is very crowded most of the time and will thin out about 5:00 PM. That is the time you can actually get some things done. I would not personally do that short trip deal, but it is possible.

It is possible. Your time estimates will work. There is a train departing Florence at 8:37 and arriving at Venezia Santa Lucia at 11:17. The afternoon/evening train will depart at 16:43 or 18:46. The train ride takes 2hrs. 40min.

William is absolutely correct. But, go anyway. You can take a vaporetto down the Grand Canal, walk around St. Mark's square and have a nice afternoon.

My hubby and I did this several years ago. It's a long day, but so much fun. We took the first train out of Florence (Euro Star) and the last train back from Venice.
WE got in about 11AM, walked around, found a nice place to eat, then wandered the streets. We took the elevator to the top of the tower, got great pictures. Toured the Jewish Ghetto. Walked over the Rialto bridge, spent some time in some of the shops. got photos taken with Carnival people, walked St Mark's. We didn't waste time on the water taxis, but enjoyed just walking and getting lost in the streets. Do buy a street map if you don't have a good sense of direction. We found everything on our own, just following street signs.

We really enjoyed the day, and don't really feel like we missed too much. I don't think I needed a night there.
Sure, we could have done more, but we really just wanted to see Venice - and we did. This day trip satisfied our curiosity. Do it!

I was going to suggest almost exactly what Ellen already wrote! I agree that you should go for it, for all of the reasons stated above.

Riding the water taxis can be nice, but I personally don't think it compares to just heading out down the streets and getting lost all over the place. You're not going to get to do everything you want to do of course, so I'd recommend that you don't plan on anything in particular. The city can be enjoyed with or without a plan!

I still haven't made it to Venice yet but will someday. Last year from Florence some friends took a day trip offered by CAF tours that they enjoyed. That could be an option if you aren't that great at navigating on your own and want to see the main sights.

I would recommend at least a two night stay in Venice to be able to take in the main sights and get a feel for the ambience, the night life, etc. But, as Larry and Ellen suggested, it's definitely doable. I would try to see St. Mark's, the Frari church, the Rialto, and areas around the Grand Canal (probably having to pass on a gondola ride and the Doge's Palace). It can be squeezed into one long, packed day, but if that's all the time you've got, you've never been there, and you really want to go, sure, why not? Just be sure to reserve seats on the morning and evening Eurostar trains to and from Venice as has been suggested.