Potential Solution
TL;DR: The butler (accidentally) did it. But really his late richy richness was trash anywy, so no tears.
Working outward from around the moment of death: Andrew (the butler) found Albert Krausse (the deceased) above decks on the ship, v drunk, waving his new loaded derringer, yelling into the storm, about wanting to "kill that no good motherfucker". Butler assumed he meant his bastard child, and wrestled with him. In the tussle, the ship heaved, the gun ended up on one side of the rail, and Albert on the other. Andrew (butler) heard voices coming up and quickly stashed the gun in a random paint can he found as he was running away, hoping not to be seen... but he couldn't quite recall WHERE so he planned to stay aboard ship and get it later and return it to where it belongs. We interfered with that plan.
In this scenario, Andrew (butler) is the father of said child. He has a loving devoted relationship with Amanda, and dotes on the child.
Importantly, in this scenario, WE DON'T EVER FIND OUT WHICH ONE IS THE CHILD. Because it's none of our damn business, and it's not actually required for the whodunnit part to be solved.
Eldest kid Sara knows about the Andrew/Amanda romance, but knows that Albert was trash, and would object, so she specifically arranged for Andrew to be elsewhere when Albert was recovering from hip surgery so that their romance/happytimes could be out of his purview until the couple were ready to come out to Albert ... that unfortunately never happened.
While the toussle on the deck was happening, David was belowdecks going down to the engine room to meet with Tony Visconti and try to resurrect their old flame, but he got cold feet and chickned out. He was going to offer Tony his earring, and only got as far as leaving it outside the door of the engine room. We know from Tony's dropped letter that it would have (might sitll!) work(ed), so now that his trash dad is gone maybe he'll get on that.
(... more coming, one sec...)
Why was Albert so angry and loading his gun? Well, the day before, he got an (anonymous? probably?) set of coordinates sent to him and (something we *don't* have) a threat that he'd want to see what was there or else...
So Albert rowed out to the GPS location, and found a buoy with a photo album duct taped to it, And inside the photo album a (thing we don't have) blackmail threat saying "pay up or I expose your bastard" or similar
Well, the photo album is clearly Etienne's - nobody else outside the family would have access to those phtotos. So Albert assumes it's Etienne making the threat (it is in my scenario) and plans to off him ... not a great plan in the drunken storm, but Albert isn't the best planner.
I'm assuming in this scenario that Albert's discovery of the non paternity was accidental - he was just interested in his family ancestry and swabbed the whole family for funzies, and the results just happened to indicate that someone wasn't made from his gametes.
Instead of rolling with it like a real father would do, and perhaps having a difficult talk with his wife about her keeping secrets like a grown-ass man, he got mad about it and complained to the one person he thought he could trust in this matter - his friend Etienne Beauregard. Who... ya know, could have been the guy his wife slept with, but whatever ... bros before wives, he thinks, ... and Etienne does some emotional labor for him.
Etienne is all "sorry bro but you need to talk to your wife, she didn't trust you enough to tell you about this and that's a problem, person-up and talk to her" ... and Albert, paragon of wisdom, promptly cuts off all ties with Etienne. Tells him he won't finance the fishmarket any more. Accuses him of being the interloper, Etienne denies this, they have a verbal row, and Albert vows never to speak to Etienne again.
He tells Amanda right then that Etienne is uninvited to all family functions and she crosses his name off the charity ball printout.
Albert calls around to encourage folks to divest from the fishmarket, lying about Etienne being shifty and untrustworthy. So Etienne tries to blackmail Albert to make ends meet. Hence, the photo album and threat ... but it's still pretty obvious that it's Etienne doing the blackmaliing. Albert didn't tell anyone else.
Before he got the blackmail threat, Albert had called the whole family onto the megayacht for a big reveal accusation drama 'venture, where he was going to disinherit the bastard, and announce the beginnings of divorce procedings from Amanda, but he never got that far.
Albert spent some time while on the yacht doing google searches for how to best go about disinheriting his child, how to divorce your wife, how to surruptitiously get cheek swabs from unsuspecting men... (we only have the first one of those as a clue)
Later, when Albert washes up on shore, Etienne persuades the authorities to let him lay on ice in his place. This is so he can check his pockets for the blackmail threat, which he finds and removes (Etienne should have this on him if we search him), but he doesn't do a complete search, so he misses the 23&me printout.
MUCH later, Etienne tries to meet with Sara - now the one with controlling interest in her late father's company - to persuade her to restore investment relations with his fishmarket.
Before she can answer, Etienne then tries to blackmail *her* - he reveals to her that he knows one of the kids is a bastard and that the father is Andrew-the-butler, and threatens to make that info public and ruin Amanda's reputation unless she pays up. Sara tells him that Andrew-the-butler saved Etienne's life - in thwarting Albert's crazed murder spree, that her dad was trash and she remains happy to restore any financial relationship he wishes to their mutual benefit. Etienne doesn't know what to do with that gesture, so he complains about not wanting a handout and storms off to think about it (we overhear a bit of this)
This means Sara would have to have seen the toussle scene, and probably she is the one that Andrew-the-butler hears and runs away from the sound of.
This covers every clue but the filet knife, thrown at Rosemary. Maybe that really is from Tony Visconti (who was nearby at the time)... who saw us get on the yacht and to whom Andrew-the-butler gave instructions to stop anyone interfering with the yacht. He MEANT to miss, and did. It was a warning shot.
(He likes us now and will prolly confess to this if prompted)
Math:
Clues Used or Explained Away: 10
Mystery Difficultly: 6
Bonus to Roll: +4