How does one sink a yacht?

She needs to have enough time get back on the dock and out of harms way

Backstroke..

Or a wet bike pops up after the yacht sinks.

I just remembered the first Pirates of the Caribbean, Jack Sparrow (Captain) sails his pram to the dock and steps off. Next seen, it's under the dock.

If you need fire and a fuse situation to give time for your subjects to get away, yet still burn and destroy the yacht, then diesel is a hard call.

Add a tender space/garage where there is an amount of petrol stored for the dinghy motors, larger tender and jet ski's plus the petrol in the tanks of these items.
Then your heroine can throw a safety flare onto a well laid out line of petrol to the petrol storage tank etc.
Or:
If you want to get fancy, grab a toaster out the galley or crew quarters, place on the petrol line, fill with dry rags and push down the toaster and wait for the fire to set off the explosion.

Yep, I have met some angry girls in my life.

If the genset was running and you cut the SW line there would no doubt be an alarm for no water supply or exhaust overheat, then there would be a hi bilge alarm, the latter would also come if you let water in from another source.

I have read books where things supposedly happen and there are errors with what's described, missing some technical details will not mean much to those reading a work of fiction.

It will of course make a difference if this is a trick you are going to try yourself on someone's boat.

This is the internet, I do not feel it is an appropriate place to discuss in detail how to carry out what will be in many jurisdictions illegal and in all downright dangerous.

If this book is a work of fiction it will no doubt include a claim somewhere along the lines of: All characters in this publication are fictitious and any resemblance to real persons, living or dead is purely co incidental.

You could add the term "scenarios described" after characters and all inaccuracies would be covered

With broken sea valves / strainers for the cooling water intake you won´t notice something is wrong unless you enter the engine room for quite some time. When you notice that the behaviour of the boat changed and it´s motions start to feel feels soft and less stable you allready have lots of water in your engine room and it´s getting difficult to locate the damage and do something reasonable to get back control of that situation.
An alarm switch for bilge water can be cheated/blocked if it´s not an electronic sensor but an old fashioned flaoting switch.

Explosion szenario: a minor fuel explosion would not damages the hull/structure of the boat seriously unless it causes a fire. Polyesther GRP burns pretty strong and with plenty of black smoke.
If a fuel explosion is strong enough to cause severe structural damage it´s also strong enoug to be seriously dangerous to everybody aboard. Generally gasoline does not have the power of typical explosives when not in a perfect ratio wit the surrounding air. You can´t control a fuel explosion unless it´s in the cylinders of an engine.

So the plot is a girl doesn't like her sugar daddy because she doesn't like the gift of a yacht? Did she want a jet? She sounds spoiled and needs a spanking. That said...

All these water intrusion scenarios are great but what about high water alarms, assuming there are no crew on this yacht and it's just the sugar daddy, then you need to get the sugar daddy away from the yacht. Go below to the "head" (boat restroom) enter the engine room, cut the raw water intake lines, go topside, tell sugar daddy "let's get a drink at that bar over there" you walk down the dock to get a drink with a name like "Davey Jones", make nice with him, then point out that you can't see the yacht he just gave her yacht anymore, mainly because it sticks up out of the water about 8 ft less. He panics, she laughs, he runs, she finishes her drink and finds a new sugar daddy to terrorize.