Where were you when you hit whatever it was. You mentioned locking thru the Mississippi... if you were on that a river, there could be all sorts of submerged stuff you might hit... I was out last week and there were lots of branches (and what looked to be large logs or entire trees) in Pool 19. After a few days, those can take on enough water to float below the surface... would be easy to hit something like that.
Again, if you're on the Mississippi (or similar river) another thought would be wing dams, although you should be OK in 17' of water.