Actually had one of these on my boat when I was growing up. Vital piece of tech.

I always remember that port and left both are the shorter word, and have the same number of letters.

Do we have any red port wine left?
- There's only a right way to find out
- That's easy. We use 'port' because that's the left side, and 'starboard' because that's the other side of the boat.
  No, no. No need to thank me. I'm just one humble man trading information gleaned from a long life of learning.
- IIRC, it was because the tiller for the rudder (with which you steer) was on the right side. (Styrbord in Swedish = steer board) As to why you have port in English, I have no idea. It's babord in Swedish, from bakbord (back board) as when steering, the left side was behind you.
  Edit: Apparently it's port because you'd dock with the left side to the port as otherwise you'd crush the rudder, which again was on the right.
My grandfather was a bomber pilot and he always remembered it by picturing himself flying north along the east coast, where all the ports would be on the left.
- Did he never have to fly back?
That's essentially how I always remember it. Port and left both have 4 letters.
- How do you explain red?
- Just like fork, so the fork goes on the left!
  I learned that from Roseanne.
- I just think of "PS" like in a letter.
Also, the old name for "port" is "larboard", which starts with an L for left.
and even, as in even numbers.
Port, left, even...all words an even number of letters Starboard, right, odd...all words are an odd number of letters.
Additionally, "starboard" has 'r's (for "right").
- ... so does "port"
Port is short. My BIL taught me that this summer!
Starbucks is always on the right side of the road!

I always picture a drunken sailor asking how much port is left. It works for me.

And port is red. Green means you're on Starboard and GET THE HELL OUT OF MY WAY!!
**red port
That's better than mine

Is there a reason they don't just say left and right?

- Still doesn't seem necessary. Your orientation doesn't matter if you always take the direction as if you were in the drivers seat (captains chair). It's the same with cars, left side is drivers side, right side is passenger (unless you're in one of those backwards countries, then left would be passenger, right would be driver, but it's still the same side of the vehicle).
Historically, boats had rudders (or “steer boards”) along the side of the ship instead of in the middle like we’d see them today. It was always on the right side of the boat, so to avoid smashing your rudder into the dock, you’d dock your ship on the other side. That means it was always the left side that literally faced the port, while starboard faced out into open water to protect that side from damage.
- TIL about steer boards, thanks!
- rudders (or “steer boards”)
  The "board" part comes from "side of a ship", as in "the board where the steer is attached".
  Same meaning as in "going over-board".
- Steer board => star board?
- whaat.
  This makes so much sense.
Then you have to say something like "your left or mine?" This removes any ambiguity, and that might matter when one is at sea.
- Your port or mine?
Also, port is red and Starboard is green. Usually boats will colour the sheets to match. It helps to identify the lines when someone tells you to ease the port sheet.

I remember being young taking my boaters safety course and having to take this section over three times. Why does left and right stop existing once you are on a floating pos?

Because left and right can be interpreted as direction from your point of view. Port and starboard are always understood as left and right of the vessel you are currently in.
- In performance arts, stage left and stage right perform the same function as well. "Stage" directions are always from the perspective of a performer, so that referring to right/left is absolute.

Is the joke here that it would be embarrassing for the captain to forget this?

Yes.

I need something like this to remember left and right correctly.

Port and left have four letters.
Mars also has 4 letters, which is useful when you're trying to remember which portal to travel through at the end of the Vault of Glass raid.
- Great, you made me ruin the rotation, I was supposed to go to Venus.
- And when you are out at sea, you have left port.
Your pointer finger and thumb make an L on your left hand.

"Red right returning". You must be leaving port.

But the red on your boat is always port.
- Maybe this one's at the stern?

You grew up on a boat?

I kinda want this in my bathroom. On the toilet.