The solution starts by identifying the murder scene:
This can be reached by being rich and getting an invite by Ronald to see his private cabin. He invites you to take a glass of expensive scotch burbon to Ariane. You go search for her first, then come back to grab the glass and see this scene. This is the picture just after the crime.
Starting from the left side, we see a reflection of Tassiana in the galley behind the wall, so she was there. We see a chair turned around. We see what look like wine or blood stains on the other chair. We see a glass on the ground, and we see an empty pedestal.
Now lets look at a couple of before pictures.
This picture is reached if you are a businessman who goes on the presentation with Tassiana. On the left you see the chair where it is supposed to be, also the right chair is free of stains, and the pedestal has a statue of Cupid and Psyche.
Here is the picture of Ronald just before the murder. He is holding the wine glass, and while it is hard to see, the pedestal of the statue is still there.
So it seems that Tassiana killed Ronald after you left to find Ariane. She used the statue as a murder weapon and use the chair as leverage to dump him in the ocean.
But where’s the proof?
As you leave to find Ariane, you hear the announcement that there are dolphins on the back bow.
This is the same announcement you hear when you are on the back bow with Ariane and see the dolphins.
It is also the point where the yacht is passing by the sailboat anchorage.
If on the second day you go on the sailboat, you can skinny dip in the anchorage with Ariane, you see those same dolphins:
Even though it is hard to see, you can also see the Cupid and Psyche statue there! If the murder weapon is there, where is the body? Bodies have a tendency to float on the ocean, so the statue could have been used to weigh the body down somehow, so it should still be here.
But remember on the boat ride Captain Jack said…
I think that answers that!