To gather ingredients, you first need to load your boat with equipment before setting it off to sea.
You can craft equipment for free in the dock and each different equipment will grant you a chance to receive a specific ingredient.
Each boat can be loaded with up to 8 different pieces of equipment of your choice, however you do not need to completely load the boat if you do not wish to.
For each ingredient you load, the sailing time will increase. When loading your boat you will be able to see how long it will be away for and the predicted haul you can receive upon return in the boat screen itself.
Once the boat returns you will be able to see a green tick over the boat, which indicates you can collect your rewards.