Ancona to Split ferry what is weather like

Is the weather warm enough to just reserve seating on the deck?
Do the seats recline?
If we need a cabin, can we upgrade once we are on the ferry?
Is a blanket provided if needed?

If you buy deck passenger ticket you can still use inside cafe, salon and restaurant so if you are cold just get inside.
Some seats recline but not all of them.
Yes, you can book cabin on spot, subject to availability.
To the best of my knowledge, blankets are not provided for deck passengers.