Jones and Betsy
Start by talking to Jones just outside the Fishing Guild bank. He will tell you the story of the Thalassus, a giant sea monster rumoured to be able to sink entire ships.
After agreeing to help him find Thalassus, he will tell you he needs an extra rower, a giant harpoon, and a particular sea chart. Ask him about each, and he will tell you to talk to 'The Guns', the Fishing Master, and to get a chart from the Fishing Master.
Try pickpocketing the Fishing Master, located near the entrance to the fishing guild. It may take several attempts, but you will eventually get the Sea chart. Next, talk to him about a smith that could help you, and he will tell you that a good friend of his is nearby smithing in Hemenster.
Head out of the guild, and head north-east to see Linza smithing under a roof. Try talking to her, but she won't really pay attention. As soon as you mention the Thalassus though, she will instantly pay attention. You will offer to return one of its teeth, should it be caught, in exchange for her to make the harpoon.
You will hand her the harpoon plans, but she won't like it and will design a new one herself, made out of metals she's engineered herself. She will hand it to you. Next, head to the Tower of Life south of Ardougne.
Look for 'The Guns' to be lifting a log and counting. He will agree to come aboard, but notes that he gets seasick. He will say that there is a potion to help him made out of lowland heather and builders' tea.
Speak to Bonafido, found laying down near some crates near the entrance to the tower. He will give you some builders' tea. Next, nice that there is lowland heather growing all around the base of the tower. Search all the plants to find the one that isn't infected with aphids.
When you finally find one, combine it with the builders' tea and talk to The Guns again to give it to him. He will offer to take you back to the Fishing Guild. Talk to Jones again to head out to sea.
Three if by Sea
Once on the boat, you will have to search shipwrecks to follow the beast's trail. The first one will be highlighted for you. You can also choose to loot the wrecks, but can only carry three items.
After investigating the wrecks, a trail will appear. You will want to the investigate the next nearest wreck. You can check the loot of any ship you want, but if you investigate too many wrong ships, then you will have to start over.
The tracks will then end...
You will be knocked overboard, and Jones was swallowed. Several hours later you will wash up on an island shore where three mermaids rest. They will explain the Thallassus has protected them for generations. You will have to talk your way out of them feeding you to the Thalassus.
Choose the following options:
- The Thalassus has eaten Jones the mighty fisherman.
- It should, he's been hunting for decades.
- Very horrid! After he's killed the Thalassus, he'll probably come after you.
- Trust me, I'm a great adventurer, I should know.
- Did I mention Jones had a harpoon with him?
- It's no ordinary harpoon.
They will finally decide to get Jones out of the Thalassus. They will give you some Karabwan to make the creature sick.
Thalassus Part 2
They will return you to your ship, and you then have to track down the Thalassus again. The Guns will then feed him the karabwan, and it will spit Jones back out.
Jones will have obtained a tooth, and you will then escape. Jones will give you the tooth to take to Linza. Linza will take the tooth the leave. Return back to Jones. He will tell you that he is now selling some Thalassus protection cream to fisherman to try out.