AC4 is very demanding. It's one of the few games you really have to concentrate on tweaking with a GTX770.
My advice is to run with VSYNC ON, assuming 60Hz (60FPS max), then tweak the quality settings until you rarely drop below this.
1) run FRAPS and the game
2) turn VSYNC OFF while testing
3) adjust quality settings so you're above 60FPS 90% of the time
4) Force "Adaptive VSync" to that game in the NVidia Control Panel
5) adjust settings again later if screen tearing occurs too often (meaning VSYNC has been turned off because you can't maintain 60FPS).
This is much easier but the drawback is SCREEN TEARING which can vary between barely noticeable and severe on games. If this doesn't bother you much, then maybe tweak to maintain at least 40FPS average.