Melee combat is not the right tool. Your people would be at disadvantage in that and nobody reasonable picks a fight in which they are at disadvantage.
1. nets
Farming is done in shallow waters where enough light still reaches to the bottom. So the height you need to protect is not that big and you can put protective nets around them. That will keep your workers safe from predators and the ‘crops’ from large herbivores.
If the nets are breached, the workers get out. They are not trained to fight, they need to direct their effort towards other skills, so they don't try it. Net-builders bring in new nets and hunters deal with predators if any got in.
You said fibrous materials are readily available, so this should be worth it.
2. shepherd dogs
In the deeper waters the main thing you can do is herding fish. So recruit a suitable marine equivalent of shepherd dogs. Probably something like dolphins.
And you always want to have a ship around. A ship with sails and oars is faster than swimming, stays afloat even if all the crew is dead tired and can have some harpoon guns mounted.
3. hunting/defense ships
Since you are able to build ship-mounted harpoon guns (the weapon is not immersed, so all the usual mechanisms—bow, torsion springs (ballista) and trebuchets—all work), use them against the larger threats. 308
Any activity when there is a risk of attack by a kraken or a large shark should be guarded by a ship with harpoon guns.
Use them in sufficiently large fleets that when kraken tries to attack one ship, the others can fire at it and kill it before it manages to do too much damage.
The bottoms need to be protected so a kraken can't bite through them with its beak. Either using a layer harder than that beak, perhaps from glass or some ceramics, or by spikes and blades so the kraken would cut itself if it tried to attack there.
Alternatively some mechanism for extending spikes when a kraken attacks could work.
4. encumbering harpoons
The old techniques for hunting whales involved having air balloons attached to them. Those would prevent the whale from submerging and create drag, both of which would help tiring it to exhaustion. You can use that too, or in some cases, you might want to try the opposite, having weights or anchors at the end of the ropes so when you throw or shoot the harpoon at something dangerous, you can get away and it will not be able to follow, because it will be dragging the anchor.
5. apropos torsion springs
With a torsion spring and a pulley you should be able to create a mechanism that would at least somewhat work as a harpoon gun underwater. Better than nothing.
6. escape mechanism
When going down to fight some predator with the hand harpoon or simple harpoon gun, you can have a rope connecting you to the ship with some mechanism that can pull you out of the water quick. A weight and some pulleys or another torsion spring should do.
That way you can wait for the predator to get close, take a shot at it and get out quickly in case you miss or if the predator does not die immediately (piercing weapons are even worse than cutting ones in this aspect) and can still swim faster than you.
7. keeping watch and planning
You don't defend from random attacks. You make sure they don't happen, or that you know about the predator ahead of time and prepare for the fight. So standing watch is important. On watch towers on the shore, on ships, and sending out scout dolphins (or whatever ‘dog’ you domesticated). 537