oak and timber are apparently not good woods to use, but if this is completely DIY, just find the dryest, lightest wood you can. I've tried driftwood from a beach, but that stuff can be hard to work with since its so scattered in shapes and sizes. floats good though. but logs would be the easiest way to go about it. some thin, light, dry logs. I'm no expert, but I know that the just about simplest way to go about it is to just basically create a matt of logs lined up, and 2 logs across to enforce them. basically this. the knots are the most important part. so get some good rope. not drygrass. that stuff is nuisance to work with, and snaps. well if your going super survivalist then you'd have to strip/bend/snap all the grass and twist it with other strands to make rope... just not worth it. rope!
use a clove hitch knot on the end and then basically just make X's and wrap more then an overhand knot (first knot in a shoe tie) just keep putting logs on the enforcement logs.
the other way to go about it would be the pressure log raft, which is basically the same thing but would use four enforcement logs. so you'd just sandwhich two logs on each side and tie the ends to the end of the corresponding log.
also know you're current very well! getting close to being pulled out to sea... scary! course there wouldn't be a sea but still. and if you can't find logs, just get some balsa wood. would float easily. hope this was remotely helpful! I'm no expert I remind you!