When does this Scene take place?: The Party is travelling by ship either at the start or near the end of an adventure.
Who are the Antagonists?: A Great White Selachii (Shark Man) and a shoal of his Remoran servants.
Why should the Party Care?: Because they are about to be Boarded.
The Party are travelling on a ship to (or from) adventure when off in the distance they spot another ship... and fire in the sky. The player ships crew (There should be enough to sail a ship, but not enough that they can suffer losses and safely continue their journey) attempt to signal the other ship.
Players attempting to Discern Reality at this point might notice a few silvery glimmers fall off of the ship into the water with small splashes and no crew on the other. Players who ask 'What is about to happen' might suspect some sort of ambush ahead.
Wether the players attempt to avoid the ship or get close to it, the only warning of what is about to happen is a crewmember (or party member) spotting silvery shapes moving below the waves...
Followed by the ship hitting something.
That something is a Great White Selachii. Unlucky for the players, it's still hungry.
There will be enough Remorans to keep the entire crew busy in the background, the only number that's important are the ones fighting the players. You wont need many because the Selachii is going to be a significant danger for the players - use the Remorans to make hard moves against players who fail rolls.
The Deep Lungs quality is to represent that the Beast-Men are having to essentially hold their breath to fight the players, and whilst they can stay above water for a good 5 minutes before needing a quick dunk to refresh, they can't stay out indefinitely without beginning to suffocate.
Play up on some of the tropes of sharks when describing the Selachii's actions - have him charge towards a player with his eyes rolled back into his skull and if the players dodge, have him go straight through the guard rail in an explosion of splinters and into the water. If a player gets seriously injured and someone fails a roll, have the Selachii go into a horrific blood frenzy where it loses its caution and is only interested in killing.
Don't be afraid to have him bulldoze someone into the water and attempt to drown them.
All whilst this is happening, describe how in the background the Remorans are Injuring/Killing the crew and, in turn are being injured and killed. When all the Remorans fighting the players die, have the ones in the background driven off victoriously by the crew.
When the Selachii dies, have the Remorans attempt to abandon ship quickly, dragging corpses with them.
And then, when everything has gone quiet and the characters have had a chance to catch their breath, point out that the ship is about to run aground on a tropical island or in known but dangerous territory.
Who knows. They might even find out why the Selachii have suddenly started attacking ships.