I think most boats (that I've seen, anyway) come with hollow-braid polypropylene. It's cheap, strong(ish), very easy to splice, doesn't absorb water, and is rot-resistant. Downsides are that it *can* be prone to UV degradation (assuming it's not UV coated), and is a bit slippery.
Two easy methods of splicing: Tape off the end, and feed it through one side, sliding it into the core by 8" to a foot. Other way is to weave it through the entire diameter a couple or three times, essentially stitching it through itself. Both methods can be whipped or stitched if you need more security.