Well, unless your MARINER is very unique, I think you mean CENTERBOARD (GRIN!> (Daggerboard raises/lowers vertically like a dagger in a sheath, centerboard pivots like a jacknife blade).
The type of cable is usually 7x19 stainless steel 1/8" diameter according to my MARINER Owner's Manual, that seems a bit small, but I checked the specs for that size wire and it has a breaking strength of 1700# or 10X the weight of your centerboard!
If you want advice based on experience, check out the Mariner Class Association web, see link on the daysailer forum page here. Or it might be worth a call to Rudy at D&R Marine (508) 644-3001, he was the Parts Dept. Manager at O'DAY from 1973-89 and now runs a little shop selling O'DAY Parts (
www.drmarine.com)