RM demo'ed a hazel and tarp boat on one of the 'Tracks' programmes back in the '90s.
It's been a while but I think the steps were -
1. Mark out an oval in soft ground - size it to the tarp you are to use as the base of your boat ensuring you leave a margin of 1' all round on the tarp to make the sides of the boat.
2. Cut a load of hazel into 2' lengths of wand approx. 1" diameter, long lengths of thinner wands and also keep all the brash (leaves and twigs).
3. Push the 2' hazel lengths into the ground to half their length (1') all around the perimeter of the oval. I don't remember the spacing but would imagine 9" - 1' would be about right.
4. Wind the thinner wands alternately between these uprights to make a short 'wattle hurdle' around the whole oval with a finished height of 12" - i.e. to the top of the uprights. RM then bound it together using cordage to make an oval 'doughnut' shape.
5. Make a lattice floor of wands by inserting them in criss-cross fashion in opposite sides (both) of the oval - do this near the top so than when the woven oval is turned over the floor is at the bottom!
6. Spread the tarp out flat and place all the brash on it to act as cushioning for the sides and floor of the woven oval 'body' of the boat.
7. Pull the oval body of the boat carefully out of the ground and turn it over. Cut off the surplus 1' lengths of upright and check carefully and remedy any pointed sticks or anything that could puncture the tarp.
8. Place the oval body on the brash on the tarp (with the floor at the bottom!) and pull up the tarp all round to form the sides of the boat. Secure by tying the tarp off at intervals to the uprights or lattice to create a waterproof skin around the base and sides of the boat.
9. The boat was paddled in the manner of a coracle - one paddle used in a figure-of-eight pattern to 'pull' the boat along.
RM demo'ed this boat with one tarp but I do remember him saying two tarps would make a better boat - one placed over the other (2 layers).
That's all I can remember. I hasten to add that I've never done it myself so I can't vouch personally for its' seaworthiness so if you do make one then it might be wise to try it first on shallow flat water with all the usual precautions.
Hope that's useful
Bob