WELCOME ABOARD!
Overall the graphics are somewhat better than SH4 and massively better than SH3. I say "somewhat" because SH5 builds on what was done for SH4. The clear water doesn't look much different, but SH5 adds a natural-looking sea foam that runs off the decks of the sub and lingers in the water. The harbors use a new rendering technology that adds a lot of details but doesn't draw much more on the PC's resources. And you can walk around the entire sub.
So graphically SH4 was a wonder to behold, and SH5 is even more so. But even a casual look at the screenshot thread would have told you that already.