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I like using ball point pens to sketch. They don't rub off and smear all over the paper and hands like graphite (hate that), but if you use light enough pressure, the line almost looks like pencil. They don't need sharpening, and you always have the option of going very dark and ink-like if you want.

Unfortunately, they don't do well if you use markers or watercolors over them.

Any recommendations for a good waterproof pen that performs like a ball point?

Uni Ball Micro--really nice and waterproof.

I like the Kaweco Sport ink roller pen (a.k.a. rollerball pen). This pen is refillable (it accepts standard international short cartridges), so I can choose what ink I put into it. Currently, mine has Platinum Carbon ink, which is a basically India ink without shellac or acrylic, and the pen-ink pairing works wonderfully.

The Kaweco Sport pen is available at some fountain pen retailers that handle lower-end pens. Online ones I know of are Writers Bloc and Swisher Pens.

I use the Fisher Space Pen, it is small enough to fit in a pocket (take off the clip) writes upside down, etc... and waterproof.