Canoes are pretty roomy, they just took food with them and supplemented that with catching fish and collecting water on the way. I've said this before, but using the term canoe is somewhat misleading to describe Polynesian and Micronesian ocean-going vessels. You're probably thinking of something like this, when in reality they were much bigger. The largest of all, the Tongan war canoes or kalia could carry up to 100 people. Similar were the Samoan va'a teles. In Micronesia, the Marshallese walap (the wa- 'boat' is cognate to Samoan va'a and -lap means 'big') could carry up to 40-50 people and food for a long voyage. This is a picture of a model walap I took at the Alele museum in Majuro. The actual one was up to 30m (100ft) long and could take up to a year to make.

Polynesians and Micronesians were the best navigators in the world until the advent of technology like the GPS. They primarily used the swells in the ocean, along with cloud patterns and migratory birds to find there way. David Lewis' We, the Navigators goes into great detail on Micronesian and Polynesian navigational techniques. In addition, you might be interested in the Polynesian Voyaging Society which seeks to recreate ocean voyaging using traditional-style vessels and traditional navigation techniques.