Would this be a good harness to wear as a...
Would this be a good harness to wear as a backup safety harness when climbing a mast on a yacht?
Do you mean in addition to a bosun's chair?
The answer is yes: it would be fine. Light and low-bulk. That said - are you climbing the mast or getting hoisted? If you're getting hoisted, you will definitely want a bosun's chair - this thing is not going to be comfortable. If you're climbing.... um - get hoisted.
As a backup for a bosun's, this is excellent.