Italy is always crazy with laws, especially Venice. This is a recent event, so things can change. They are trying to get day-trippers arriving by train to contribute to the economy, to keeping Venice clean and safe, as well as the tour ship crowds. The tax will only be 3 euros this year, then next year they will charge by season. May is high season.
That said, this tax is really designed to target people who don't stay in Venice. Day trippers who come to the city and "morde e fugge," or take a bite out of the city then flee, after making the life of locals intolerable. There is no intention to make people who stay in a hotel pay this tax because hotels already charge the Tasse di soggiorno, or travelers tax, and that is 3 euros per room, and you don't have to pay it after 5 days.
Since you are staying in a hotel, you will be OK.