You need $57 before anything else happens on the starting island: $45 for the Knife and $12 for the Beer. The Beer goes to the lighthouse keeper, the empty can he hands back is the bait that summons the Spider Crab, and the Spider Crab Shell you bring him afterwards is what gets you the boat keys.

The lighthouse keeper sitting on his wooden bench below the lighthouse door, the NPC who takes the Beer and the Spider Crab Shell

Raise the first $57

The keeper sits on the bench under the lighthouse. Pick up the clams lying along the shoreline, look at him and hold Q to throw each one at him; every clam pays $1. That is slow, so turn the first few dollars into a rod as soon as you can.

Buy the Crab-Fishing Rod for $3 at the island shop. With no bait fitted it fishes with Ham, which brings up Brown Crab ($3) and Shrimp ($5). A $1 Hot Dog on the same rod switches the pool to Rock Crab ($7) and Lobster ($9), so the Hot Dog pays for itself on the first bite. Cast with the right mouse button and reel with the left.

Kill each catch before you carry it to the keeper: a hooked creature fights back, and a dead one can be thrown at him with Q like the clams. Once you own a weapon, small catches die in a couple of hits and the money comes in much faster.

Gear up for a melee fight

No guns are sold on this island, so the Spider Crab is a melee fight. Your choices from the island shop:

Knife ($45): the reliable pick. Its damage is clearly higher than bare fists, and it is enough without any upgrade.
Brass Knuckles ($24): cheaper, and fine for finishing small catches, but each hit does less.
Sharpening: the anvil by the shop raises Knife damage one step at a time (one sharpen took it from 16 to 18 damage in a recorded run); each step costs more than the last. Optional, but two extra damage per hit noticeably shortens the fight.

Before you summon the boss, eat until the heart in the bottom-left corner is full and keep a few dead creatures in your inventory or on the ground nearby. Three or four hits from the Spider Crab can kill a fresh character, and eating mid-fight is how you recover.

Summon the Spider Crab

Buy a Beer for $12 at the stand beside the keeper.
Hold the Beer, face the keeper and hold Q to give it to him. He drinks it and hands you an Empty Beer Can.
Equip the Empty Beer Can on the Crab-Fishing Rod (press B to switch bait) and cast from the rocks near the keeper.
When the name "Spider Crab" and a red health bar appear at the top of the screen, reel it in and pull it out onto the rock.

First-person view holding the yellow Empty Beer Can next to the seated lighthouse keeper, with the Inspect F prompt on screen

Fight: dodge the charge, punish the stun

The Spider Crab runs at you in a straight line. Sidestep it and it ends the charge dazed, with yellow sparks circling its body. That stun is your only safe window: get behind it, land three or four hits, then back away before it recovers. Sprint with Shift to make room for the next charge.

The Spider Crab stunned with yellow sparks above it while the player holds a Knife; the red health bar and the white escape bar under it are visible at the top

Fight it on dry rock, away from the waterline. If the crab touches the water it springs much higher on its next jump, which is far harder to dodge; if it stays on the ground it never gets that height.

Watch the thin white bar under the health bar. It is the escape timer: when it runs out, the Spider Crab leaves and you have to summon it again with another Empty Beer Can. Patience wins the fight, but if the white bar is about to pass the remaining health, take riskier hits to finish it.

On Normal the Spider Crab has 750 health. Easy cuts that to 75% and Hard raises it to 125%, and every extra player in the lobby adds health too, although friends with knives still make the fight much shorter.

Hand in the shell, not the meat

When the crab dies it breaks into meat chunks and one Spider Crab Shell. You get one chunk per player plus two (three when solo), each worth $10, and you can eat or sell them. Only the shell finishes the quest. Pick it up, throw it to the keeper with Q, and he gives you the boat keys.

If the keeper does nothing when you throw an item, check that it is the shell. A meat chunk looks similar from a distance and never advances the quest.

Buy the Radar before you sail

The boat keys alone will not show you where to go. Buy the Radar for $10 at the stand next to the keeper and hold it in your hands: a green dot appears near the top-right edge of the screen. That dot is Island 2, the Forest, to the northwest. Drive the boat toward it and stop to check the Radar whenever you are unsure of the heading.