This concept is explored in the second sequel to "2001: A Space Odyssey", "2061", albeit without approaching relativistic speeds.
Clarke explores the idea that in a future where fusion reactors can be built, a spacecraft can have a fusion-powered motor for which the fuel is simply water. In the book, a spacecraft travels from Earth (or maybe Mars, it's been a while since I read it) to Jupiter, and does it the "brute force" way by simply pointing at Jupiter and thrusting at 1G until they are half way there, and then turning around and slowing down at 1G until they arrive.
This has the bonus side effect of allowing the spacecraft to have normal gravity inside it.
Clarke explores the idea that in a future where fusion reactors can be built, a spacecraft can have a fusion-powered motor for which the fuel is simply water. In the book, a spacecraft travels from Earth (or maybe Mars, it's been a while since I read it) to Jupiter, and does it the "brute force" way by simply pointing at Jupiter and thrusting at 1G until they are half way there, and then turning around and slowing down at 1G until they arrive.
This has the bonus side effect of allowing the spacecraft to have normal gravity inside it.