They would have to lug it all somewhere, wouldn't they? Either way, they've got to do the work and take the risk.
Yeah, they could have done that. That's assuming a random, unthinking perp.
A local, thinking perp would have used a boat. It's just the most effective way. Let me try to paint it for you - how it's not that hard.
Wrap up your evidence in a tarp; not necessary for the bike. Weight the tarp. Put it all in the boat when no one is looking and conceal it, Hook up your boat to your truck, drive down to a private or public landing somewhere around Butte la Rose. Drive up river for an hour or so. Enjoy the scenery. Bring your fishing poles, too, to make it look good. Ride past the junction of the bayou and the Pilot Channel, where the current gets fast and deep. Ride past the little camps on the left bank, so as not to be seen. Dump everything overboard. Ride back for an hour or so.... pull the boat out of the water.... You are done!
If the bike did somehow float, then that was his only mistake.