Leave the ferry - head to the right of the stairs that lead to it, and then go along the shore until you reach a large platform, with a tower that resembles a lighthouse on top of it. The captain will be waiting for you by the door to the shed - head inside without further ado.
(You can also try to use the stairs to climb the platform first and speak with the fisherman. If before that Kate talked with the craftsman in the big Youkol tent about the octopus' sculpture and listened to the local legend about a monster you will receive The Valsembor horror achievement. If up to this point you have managed to complete four actions in advance that has an impact on the progress of the game then this one will be your fifth and for that you receive The scriptwriter's nightmare achievement - more details for this achievement can be found in the dedicated chapter).
In the storeroom from take the diving suit from the hanger by the door - in order to do that swipe (by moving the mouse in the appropriate directions) the other clothes aside, so that the large suit underneath them becomes available. You should also grab the empty oxygen tanks, that are standing to the right of the hanger. Take a look at them in your inventory - you will learn, that the required oxygen pressure for said tanks is equal to 180 bar.
Go further into the shed and take the quite archaic diving helmet. You should also take a look at the device on your left. When in the close-up, install the oxygen tanks into the relevant positions (under the first levers on the left).
Fill the tanks with oxygen
Now take a look at the metal box, that is hanging on the left. While in the close-up, turn the knob to the right of the circular indicator, and set the pressure to 180 bar. Press the green button at the bottom of the box - the device will start to shake slightly. Zoom in on the tanks. First, pull the grey handle down (with the correct mouse movement) for safety, then pull the red lever on the left down (with the correct mouse movement), and pull the right one after a moment, in order to fill the tanks.
When Kate collects them, move to the changing room in the right corner of the room (the open door with the lifebelt, the active point is deep inside, in the middle).