Continuing...

If you use a straight stalk of bamboo you'll get limited life from your boat. It is not impossible to seal the insides of each joint of the bamboo stalk, but it is difficult to do it completely, and time consumng to try. If you want to try this, I'd suggest boring two small holes in each joint and using a hypodermic syringe to fill the cavities with a thinned mix of 80% epoxy with 20% acetone. Let that soak in for a minute, then use the syringe to suck out any excess--which you can inject into the next joint.

Otherwise, go back to the techniques used by rod makers. Use a powersaw to rip thin strips from stalks of bamboo. Plane them smooth, and laminate them (using epoxy) to build-up the parts you need. You can make long, straight strips for stringers, or you can bend the pliable strips around forms to make beautiful laminated frames of any dimension. The abundance of epoxy used in the laminating, and the two coats of epoxy you'll seal each part with will make them waterproof and give them a long life.
Frankly, I think it would be faster, easier, and cheaper to buy the one hardwood board. All the epoxy you'd use with bamboo can really boost the cost.
One other possibility. Frames made from thin strips of bamboo which are lamnated without glue,but lashed together with cane,and wrapped with cane. Seal the whole thing with a saturating coat of shellac, then cover with several layers of varnish. Who knows. It might just work for you.
PGJ