That is from the dock that blew away during a storm this summer. A coworker had his boat there and it was on that dock. Luckily he had only minor damage to his. Others were not so lucky.
The only problem I would have is how to cut it to make it look nice. Although if it were a big enough block you could just carve it out and leave a shell.
Foam , being a homogeneous material , can easily be added to or subtracted from ... The foam blocks I bought were nasty on the outside , but pristine inside once sliced open with the Hot wire ... For the price (Free), leaves plenty room for “experimentation" ...