Hmmm...what's the difference between SF and F? What a fine question. As a writer of both, as well as other things, for a tad over 51 years, I think I have an answer for that. An answer that works, too!
First, Science Fiction comes in two basic forms. One is hard (meaning accurate) and the other is soft. Hard SF is driven by several things. Characters, philosophy, psychology, and, above all, science. The science in hard SF is projected forward if the story occurs in the future, but it doesn't have to be. In hard SF there is speculative, "What if?" and non-speculative. What I write today that involves today need not speculate on anything, but it usually does. All future based SF, both hard and soft, contains an element of speculation, obviously. So, whether speculative or not, hard SF has the science riding in the front seat and often shares the driving chores. Soft SF, has the science riding in the back seat and it may shout out directions to the drivers, but the drivers are characters and plot.
Science Fiction means what it says. The science employed must be based in real science. You are free to stretch the envelope all you want, even to the point where it rips, but your science had better be defensible. That is, the writers had better know what they are talking about. A sound and working knowledge of science must be in the writers portfolio or he/she should know someone in the science being used to keep it as real as the plot will allow. Believe me, there are readers who know their science and any straying from the path of possibility needs to be defensible. This holds for both hard and soft SF.
Much of the stuff stocked on the shelves under "Science Fiction" is NOT science fiction at all. Science Fantasy, filled with technobabble is there, too. You will also find pure Fantasy there. As soon as anything like magic, and other things associated with fantasy show their ugly faces, the writer has slipped from SF into the realm of Science Fantasy or plain Fantasy.
So, does one need to be a scientist to write science fiction? The answer to that is, "No." But I can tell you that it certainly helps.
"The speed of the brain is inversely proportional to the speed of the mouth squared."