Good advice from PK ... EXCEPT.
The Alpine Bod harness is a nut crusher. Oh. Oh. It Hurts Us.
Get yourself a nice swami/leg loops combo instead of a diaper harness. You can get a really cush one
for $35. Bosun's seats disappeared from wall climbing when good harnesses replaced the Bod ball-manglers. I've spent 8 hours at hanging belays in a swami harness; wasn't fun, but the feeling came back eventually.
Also, I still strongly advise against the Figure8, munter hitch, or tube-style rappel plan. In harbor, with help, okay. At sea, you could be dashed to death unless you had someone working a Fireman's Belay on deck. I prefer any mast-climbing system to be operable by one person -- because maybe the knockdown that just fubared your spreader also busted your partner's ribs....
To that end: fix both ends of the rope with one goodly armload of slack in it. Jumar up; jumar down. It's exactly as easy to go down with ascenders/prussiks as to go up. Motions are identical. And it's a damn sight safer than rapping.
No additional equipment needed, no dangerous transfer from one system to another. But yes, tie off short or use a running prussik. If you want to be extra safe, tie off short to a second
line, like a spare halyard, in case the rope you are ascending fails.