dulfersitz wrote a review of Beal Rope Marker on September 23, 2013
Familiarity: I gave it as a gift, but have feedback to share
Bluewater has a good discussion on their FAQ about middle markers. They recommend any non-solvent based marker, particularly Sharpie's Rub-A-Dub laundry pen (which you can get for $2 on Amazon).
http://www.bluewaterropes.com/ and click on FAQs at top. Here's what they say:
"Marking pens are fine to use on ropes as long as they are water based laundry markers. Years ago solvent based markers were the norm. Some of the solvents used in these old pens could reduce the strength of the sheath strands marked. These days most pens are water based so this is not as much of an issue as in years passed. We recommend a Sharpie 'rub a dub' laundry marking pen."