certainly there are water taxis available, but as 17thousandkm says they will cost around €100 - good value if there's six of you, not so good if there's two [but worth the money]

I thought that you might be able to use the alilaguna public boat transport, but checking the timetable that's what doesn't run until after 8am.

Cheapest way is probably a water taxi or vaparetto (check timetable in 17kkm's post) to Piazzale Roma and then public bus from there (have a look here for some good info.