You probably heard that it is not allowed in ship battles. I highly doubt it is illegal server-wide.

Put as much obsidian as you’d like on your ships. The only time they’re not allowed on ships is in most ship battles Max hosts. But I wouldn’t worry about that unless you are making the ship for that specific purpose.

But yeah build away, make her a strong ship.

Yeah, not illegal (as you seen) just incredibly expensive to produce and stuff. Great you’ve got a plan and all, I would only use it in dire situations though or as a deterrent as I wouldn’t want to be sailing and suddenly the Rambo of the sea (JUSA) jumps in, pulls the pins on 3 tnt, triple backflips out of the ship in the form of Tom Daley only to not look at the explosion behind him as he swims away.

Yet, if you wanted to use such a weapon, one of the problems being that ships are slow, and bigger shipa require a higher rank (usually a ship that even has obsidian shouls be a manowar+). Also, a ship requires mainly two things: wood/whatever material you are using, and wool. Obsidian is considered as armour, and the wood and wool need a percentage of the ship, or else the ship will sink. In conclusion, be careful about how much obsidian you are using, for power should not be abused!                      .