On October 11, 1997, at 1030 central daylight time, a float equipped Piper PA-18-150, N9691P, operated by a commercial pilot collided with debris and nosed over into the Missouri River while landing on the river 20 miles southwest of Williston, North Dakota. The pilot was not injured. The airplane was substantially damaged. Instrument meteorological conditions prevailed and a flight plan was not filed. The flight originated from Poplar, Montana, with an original destination of Bismark, North Dakota. The pilot reported that he was ferrying the airplane from Alaska to Florida when the accident occurred. The pilot reported that he was following the Missouri River en route to Bismark when he encountered 700 to 900 foot ceilings and visibilities of 3 to 5 miles which eventually deteriorated to 500 to 700 foot ceilings and visibilities of 3 miles. He reported that due to the weather conditions he decided to land in Williston and wait for the weather to improve. The pilot reported that while flying down river from Williston, he noticed a "significant" amount of debris floating in the river with no suitable areas on which to land. He reported that he continued toward Williston and when 20 to 25 miles away, "... I came upon a long straight stretch of river that was suitable for landing and opted to set down here and not continue the flight." The pilot reported that upon landing he added power to maintain the airplane on step so he could taxi to a boat launch area. He reported that as he added power the "... floats impacted something in the water, perhaps a submerged log floating downstream, and the aircraft pitched dramatically nose-down into the water, and yawed to the right toward the shoreline. I stood on the left rudder to attempt to straighten directional control as the plane plowed into the water, and the right float impacted rocks on the shore before I could arrest the directional control." The airplane began sinking at which time the pilot exited the airplane. He reported that as the airplane began to roll over he jumped into the water and began to swim. He reported that he had trouble swimming because he was wearing hip- boots. Once he was able to remove the boots e was able to swim and a bystander helped him to shore. The pilot reported that he did not obtain a weather briefing prior to the flight, but had attempted to contact flight service en route to no avail.