Challenge: Destroy the box
Can you do it?
It's one particular memory, the one where you fetch paintings for Maria in sequence 1. The challenge is to destroy the box with paintings you have to bring back.
Just try it, and you'll see how hard it is.
Are there roaming restrictions? If not you could just walk into the river...
No, there are no roaming restrictions.
And trust me, it's not so easy as to just walk into the river. I tried multiple times, none of them worked.
Also, I discovered something weird. If you run into the water and 'accidentally' fall in (like you're just running and then you drop in the water because you have momentum), the box appears to register as in your hands, despite it being on the shore. The goal on the minimap is still the Auditore home (if you're not holding the box, it's the box). This state lasts while you're on non-standing surfaces, akin to hyperblend/superblend. However, it's unusable as you can't get anymore than 25 metres away from the box.
Like I said, it's pretty hard to destroy the box. 
Like I said, it's pretty hard to destroy the box.
Because Leonardo's paintings are immortal? 
Apparently so. I'll see if I can upload a vid with some more details. It's already recorded. (Along with some fun stuff involving Federico...)