I'd recommend a test first....typical boaters luck leads me to believe it will dye the emb. then as soon as it gets wet, if will run and permanently dye thoe precious nauga's that valiantly gave their lives so you good use their hides for your seats...
Let me ask you professionals this question....I have that out dated teal color as shading on my stiching. Can I go over it with a different color (like grey) to change the color of it? Or are they not powerful enough for that.
I friend of mine's wife works for JoAnn Fabrics, she is going to try one of the markers on white vinyl and see what happens, if it stains, if it wipes off..ect..
The fabric markers work, they will fade in time but have not blead. I use it on my ZX emblems on my seats the red fades and the fabric markers bring them back to life. I hit it with a little scotch guard after it tries.