[@Genni][@Senor Herp]

Vasha isn't quiet a Tomb of Horror's Lich, but the Castigati have long had a hard time dealing with him... Largely due to the aforementioned Greek Fire and the Deep Ones that live in the sea around the Isle of Shipwrecks. There is a system in place so that the Fishmen under the waves can recognize ships that are allowed to travel to Tushiena and what route they are meant to be on; If a ship doesn't have approval or is found sailing on a route it's not meant to be, it gets sunk pretty damn quickly and the Fishmen aren't known for taking survivors for questioning.

It stops the Castigati and other powers from launching an invasion fleet, so the demonslayers attempts are limited to trying to sneak onto accepted ships to get to Tushiena... at which point they have to sneak through customs, try and avoid detection by the powers that be in the city and enter Vasha's private keep... any that actually get -that- far are now facing a very old, very powerful lich who likely isn't very pleased to have unwanted house guests and his personal guards. 

To be fair, Vasha likely has made arrangements with his neighbors to allow 'approved' ships to sail through his waters without being sunk into two categorizes; Those who sail and trade with Tushiena itself and those that merely have to sail through it's waters to go elsewhere.