Not sure where to start but bear with me...
I grew up just a few miles from Denton Creek, and until a few months ago lived a few hundred yards from it. I have fished the creek countless times since I was a kid and few time in the last year. I have been on the creek in a boat and have walked in from Denton Hwy. On the east side of the Hwy is a mountain bike trail that skirts the creek that I have ridden dozens of times so it would be safe to say that I am extremely familiar to the area. The creek feeds into Grapevine lake and flows basically NW to SE. When the creek is it at normal levels it has mostly sheer 10ft walls of dirt and clay on both sides and murky water, blind curves seemingly throughout, and Hundreds of tons of debris like trees and brush that makes navigation hazardous at best. In a few places west of Denton hwy (377) there are underwater rock ledges that are impassable in boats when the water is low. The depth varies throughout the length of the creek but mostly is about 3 to 7 ft, but in a few areas can reach 15 feet. After a rain the creek is MUDDY, with zero underwater visibility and the debris moves around. I have never seen an alligator in the creek, but I have seen some rather monstrous alligator gar, feral hogs, snakes, bobcats and coyotes.
I believe that he probably went overboard and likely drowned due to a previous medical issue or by striking his head on some debris. It is very likely that the boat hit debris a threw him from the boat. That is because he was sitting at the back where the tiller (steering and gas) are located on this boat. When a boat strikes debris like log or tree it kicks the front of the boat up and then violently kicks the rear up in the air like an unbalanced see-saw, I know this from experience that I don't care to do again . Regarding how far the boat went onto shore, it is absolutely possible! Last year we were in a much larger (28ft) and heavier boat in the Mississippi river Delta and hit a sand bar at about 35mph and we slid at least 30 feet onto the sandbar. Everyone in the boat was thrown onto the deck of the boat, not out of the boat and we were all standing up.
The water in Denton Creek really cannot be effectively searched by divers because of the water clarity and the debris. IMHO it is highly probably that he drowned and is caught underwater in some trees. Of course this is all if he didn't plan his own disappearance but I don't know if I believe this to be the case. My question is: what happened to the fishing poles? You don't go up the creek for boat ride, its too hazardous.