Ulfhedinn:
I understand what you want to accomplish. I have set out on the same path and never quite got there, not because I failed, but because I failed to finish.
Sharpies, silver, black and otherwise are nice, but for sure on your fins they can be rubbed off with simple fingernail polish remover (acetone.) Also, unless you are quite an artist, it will look sloppy and hand done. BTW: "Permanent" is a myth. Even the industrial strength markers are not necessarily permanent.
The path I set out on was screen printing. The U-Name-It or fabric paint is great stuff. Even some of the dive toy supply houses sell it to paint on dive gear. It is very durable, and has some build so it will not just rub off. However from the tube, you have less control than you do of a sharpie, and you will wind up with a sloppy hand made cheesy look.
So, what I was thinking is a combination of screen printing, and the fabric paint. There are kits available from SpeedBall, and I am sure others. I am not sure if the fabric paint will pass through the mesh of the screen printing kit, but you can improvise. Arts and craft stores are good sources.
I suggest you practice, practice, and practice some more until you get it right.
I would start with some fine mesh screen on a cheap embroidery hoop. Acquire (without your wife's knowledge) a silicone spatula or a some other suitable squeegee. Attach the mesh to the embroidery hoop and stretch it tight. Draw the outline of your logo on the mesh. Figure out how to block out the mesh that is not your logo. Modpodge, or some other glue or silicone sealer will be fine (I think). Once this has dried, press the screen onto the fabric/surface you want the logo on and squirt the fabric paint on it. Keep it pressed down tight or you WILL make a mess. Squeegee the paint through the mesh and squish it through there pretty hard. Slowly pick your screen straight up and you will have a professional looking logo. It will be wet, and can smudge, so let it dry undisturbed for a while.
Essentially this is the method commercial screen printers use to put logos and fancy stuff on shirts and hats etc. They have nice fancy machines to assure it goes where you want it. They also use ovens to dry the paint (they call it ink.) Multiple colors will require multiple screens and registration so everything lines up, making the process much more complicated, so I would stick to one color.
The main thing is have fun and approach it deliberately, and in not too big of a hurry. I guarantee if you rush it or try to be a cowboy you will make a mess. I guarantee nothing else. Test each step to see if it works before moving on to the next step.
Again have fun, and post some pictures when you are done.