Re: Essential knots?
@jack: bowlines (with a safety hitch) are perfectly fine for ascending a mast really... especially if you're being hoisted up. The weight delivery to the knot is constant and doesn't necessarily take on a sudden load as with climbing and sustaining a fall.
As a side note, I think it's interesting how most rope companies, like New England Ropes, sells to different populations. They make low stretch lines for halyards, and then very high stretch ropes for climbing. So the low stretch line that will break you in half in a fall climbing is great for a halyard... The super stretchy rope for climbing would be a POS halyard. Either way they'll both cost you an arm and a leg.