In order to get into the cockpit of the Vasagatan you will need to craft the bomb at the workbench found on floor 1 of the boat. The bomb will require four items to make.
The gas tank can be found on floor 1 in the kitchen.
The bullet can be found on floor 2 on the bar. It is easy to miss because it looks more like a bottle of alcohol than a bullet.
The electrical wires can be found in the safe on floor 2.
The lighter can be found on top of the bed on floor 5.
You shouldn't have too much trouble finding each item. The game makes it especially easy by making each item you can pick up glow white. It can be very difficult to see in the Vasagatan at night which is why you find the recipe for the headlamp at the Radio Station, however, you can just sleep so you don't have to waste materials on the headlamp. The Vasagatan is so small that, unlike Balboa Island, getting back to your boat quickly to sleep is no problem.