How does one sink a yacht?

One simply has to overcome the positive buoyancy and their pride and joy will vanish below the waves in short order.

Basicaly the trick with boats is that the water is outside...

When writing your story, have your clever and resourceful protagonist log onto a yachting website so she can ask how to sink a boat.

I watched a yacht across the canal from me sink, no help needed.
She was a wooden 50 foot ketch built in 1964 and had been sitting in "storage" for a few years with only basic maintenance.
Then she sprung a plank. That means the fasteners holding the plank onto the frames let go, either due to corrosion of the screws, or rot in the frame holding the screws.
The whole process was automatic, no help needed.
Most boats will sink on their own if you wait long enough.
To speed up the process you can turn of all pumps and let a garden hose do the job, a slow but sure way of sinking anything from a canoe to a cruise ship.

Come on guys, she sang your praise and with the exception of Dag, she got smart @$$ answers from the lot. Erin, most (all) of these guys have spent their careers making and keeping things afloat. "Sink" isn't in their vocabulary. There are many ways to sink a boat, but there are variables, such as the hull material and machinery onboard.

1. If it's a wooden hull, drill a few holes in the hull (subsurface, of course)
2. Remove the desalinized hose clamp from the fresh water maker
3. Cut the hose for the generator cooling intake
4. Remove any through hull fittings
5. Tell the wife you're having an affair

Systemo Capitano Schettino, he has good experience.

Okay Carl, I'll be nice, any of the above, plus turn off the batteries and power supply (no bilge pumps or alarms, assuming your heroine doesn't want the crew or others to come running to the engine room and quickly fix the problem.)

Ask the kids that sank Yogi.
Ah, That's it. If it's a newer boat with side openings or a big stern opening in a follow sea.. Open it up (in the Med, in a storm really makes the news). Water does not stay out for long.

For reality, newer super yachts are built to geographical (political) codes. These codes usually dictate how a ship is built and requires some water tight doors in some areas. These should save the ship in case a side door is open in a storm (Yogi still went down). You could omit this technicality from your book, or include it and explain that a relief crew that did not know anything about ships or weather were in charge and left some doors open.

http://www.yachtforums.com/threads/proteksan-turquoise-superyacht-yogi-sinks.18129/

Redundancy is the name of the game with yachts. So sinking one isn't all that easy if there's someone aboard who wants to stop it. Fire extinguishers are located in many locations, as well as automatic systems in the engine spaces. Almost any thrugh-hull will have a seacock that can be closed. So cutting a hose won't do it. Explosives are effective and dramatic, but it's been done a million times in movies. Not dramatic, but if you poured acid on the shaft it would seep into and disolve the shaft log and packing. Again, not very dramatic, but effective and almost impossible to repair. Of course the bilge pumps would have to be disabled, and even then the motors could be turned into bilge pumps. Reduncnacy.

Oh wait, there more;
Alter the auto pilot from below. These fancy & remote compasses are usually hidden low in the ship. A magnet or lighter next to the AP compass can make the boat drive up on the rocks or a pinnacle.

Hire an Italian slob to drive your ship on the rocks.

RCRAPPS, Were you driving this one?

http://www.oceanofnews.com/5m-112-yacht-nearly-sinks-hitting-rock-off-st-thomas/

Not THAT one.
Go try to sink this one though.

A fire axe against the sea valves for the engine cooling will do. as long as the valves are suffciently damaged there are very few ways to stop the water coming in and the diameter of these water intakes is most often too big for the bilge pumps.
No explosives needed. I just hope this won´t be abused for an insurance fraud. But then again everybody who plays with boats knows exactly what he/she doesn´t want to see damaged aboard...

If it s going to happen at the dock, forget running running aground etc

Fire is messy and can quickly get out of control.

The most plausible scenario would be to go in th engine room, open everyt sea strainer she can find (main engines, generators, air con, water maker etc...). Remove the handles on all sea cocks so that they can't be closed (will require a wrench)
Locate the bilge pump fuses/breakers and pull them out.

That shoul do the trick.

Glad you are asking, I absolutely hate reading a good book only to have the whole story line sink because the writer didn't bother doing minimal homework!

I was just about to post "didn't a yacht sink sometime in the last year or two because the transom door was left open?" when I read rcrapps post.

The OP said the boat needed to make it back to the dock, so I left out cracking the sea strainers. That would only leave a few minutes before an engine became ballast.

The boat may well be at the dock already. OP said the protagonist has to be able to "get back on the dock". OP also states that the antagonist has to be able to get away, but not save the yacht. The combination leads me to infer that the boat could be docked, and (maybe) just needs to sink in place.