Okay, so the thing is that the heart of the sea has nothing to do with unlocking stages, it's a "side-quest". To unlock stage 2 you need to gather 200 "points" by first of all killing 100 deepling guards for 100 points. And to get the remaining 100 points you will have to use a soul net on deepling spawns (there are 22 spawns which you will have to use your soul net on 20 times), when you have done this you will get the remaining 100 points. Note that these points are not individual and can be gathered by a lot of people during the day. At server save the points will reset.