The Hunt for the Sea Serpent Quest/Spoiler

Required Equipment

 * 5 Fish
 * 5 Green Perch
 * 5 Northern Pike
 * 5 Rainbow Trout
 * Bait

Method
Find capitan Haba in Svargrond ship (this one to the south) and ask him about mission. He will ask you to bring him 5 Fish, 5 Green Perch, 5 Northern Pikes and 5 Rainbow Trouts. Once you deliver them, you will be able to swim to open sea, just say 'hunt'. You will need some Bait to finish quest, buy them from capitan for 250 gp each. Next, go to the top of mast and use telescope till you receive a message about little dot in the sea. Report it to capitan and use Bait on fishing net. Again get to the top of the mast and use telescope, message which will appear should be reported to capitan again and another Bait should be used. Again use telescope and again report message to capitan. After some time you should be able to enter underwater. Don't forget Helmet of the Deep.