For most, going west meant traveling by both wagon and boat.  But who
was the guy who figured out how to do both at once?

The Conestoga Wagon had characteristics of both a wagon and a boat.
James Logan is generally credited with having invented the Conestoga
Wagon, or at least with having popularized the name.

"Both ends of the wagon were built higher than the middle. A high and
rounded white canvas roof could be put on the vehicle, making it a
covered wagon. Wheels with brad rims prevented bogging down in mud.
These wheels could be removed and the wagon could be used as a boat. A
version of the wagon, the prairie schooner, was used by westward
heading immigrants later in the 19th century."

Cooperative Educational Service Agency #10: Prairie Transportation

"James Logan... is credited with being the inventor of the Conestoga wagon."

Cynthia Swope: Within the Vines
http://www.cynthiaswope.com/withinthevines/enterCOMPLETE

Google search strategy:

Google Web Search: "conestoga wagon" invented
://www.google.com/search?hl=en&ie=UTF-8&q=%22conestoga+wagon%22+invented

Google Web Search: "james logan" conestoga
://www.google.com/search?hl=en&ie=UTF-8&q=%22james+logan%22+conestoga

I hope this is helpful. If anything is unclear or incomplete, please
request clarification; I'll gladly offer further assistance before you
rate my answer.

Best regards,
pinkfreud