While true that you can't turn a, say, tanker into a battleship, you
certainly have the right by international law to protect yourself, the
cargo and the vessel.
If that means carying anti-tank weapons, automatic rifles or hiring
mercenaries to protect you, your cargo and your ship, that's perfectly
legal.
Absolutely, but protecting yourself is only a band-aid. If you want to
drastically reduce piracy, you have to take it to them, keep them locked
up in port. A few ships blockading their home ports, is worth many ships
on the high seas. However, under international law, a blockade is
illegal, and whether we like it or not, we do have an obligation to
follow international law.
It's not illegal for a nation to defend its merchant ships by patrolling
the seas from the and by having gunships accompany the merchants.