My pc is brand new, I was gonna buy the full game but then I though I should check out the demo before I buy it. It was really laggy until I built a wall blocking the sea, so the water/sea effects are lagging my pc.
In the Steam version, you can change the water quality and disable real time reflections. You can also enable motion sickness mode that will keep the sea calm which improves performance somewhat. You can also change overall graphics quality and quality of textures, though I'm not sure how much if any they effect the sea/water.