If you happen to get a hole in the raft, you want to be sure to have something to patch it with in the field. Not a problem on class I waters. Just make sure it doesn't get drenched. If you have a general purpose raft, you'll eventually get a hole in it. Believe me, you will.
Holes are usually small and can be easily repaired in the field.
Unless the hole is fairly large, it will be difficult to find. You can look for, listen for, or use a hand to feel for it. You can also submerge parts of the raft in water and look for air bubbles. To repair small holes in the field, you can use a full fledged patch or simply cover the hole with an ample amount of glue that came with your repair kit. Before repairing, be sure to deflate the raft enough so that air is not being forced out of the hole.