You will begin by only seeing the boss’s tentacles briefly strike the control room. Exit this room and continue your way to the upper deck. As you proceed through the hall, you’ll find two of the boss’s tentacles staying in the wall.
Be wary – if you get too close, you’ll be grabbed and launched out of the ship. Use a Skyward Strike to get rid of both tentacles, then go through the doorway and upstairs.
Here, go along the hall and avoid the barrels; they fall in a left-to-right pattern that is easy to predict. Continue along to the top floor of the ship. Use the nearby Bird Statue, then go through the door to the deck to find your next boss (the actual fight).
Now, for the first phase of the actual fight. You’ll initially have to Skyward Strike some of the tentacles that break through the deck – whenever this thing starts to happen, always have a Skyward Strike at the ready. Move around the whole time and avoid being grabbed and hurt by a tentacle. Slice four down to continue.
Tentalus will then rise from the water on one side of the deck. Ready your Bow and aim at the boss’s yellow eye, shooting when possible. Note that you can still move around while aiming with the Bow (so as to avoid the arm slams).
Once you hit the eye, Tentalus will slump onto the deck. Run up to him and start slicing the eye directly. You’ll repeat this, starting from the above paragraph, until Tentalus’s eye is damaged enough.
Once it is, the second phase begins. The tentacles will literally ravage the deck, similar to Bowser from Yoshi’s Island, and you’ll have to dash jump on a nearby crate to the higher ledge. Tentalus will focus on using its head tentacles to attack.
Slice the tentacles until you can find a time of relative peace, in which you can hit Tentalus’s eye with an arrow. Once this opening is found, blast an arrow at him, then start slicing his eye. Repeat the process if needed until dead.