Something I've noticed in fantasy novels is people curse by what's most important to them.

For instance, in Anne McCaffrey's Pern novels, dragons are all important to the survival of humans, so one of the most frequently used curse words is "Shells!" In reference to dragon eggs, of course. In Anne Bishop's Ephemera series, the world has been shattered into pieces, and magical people known as Guardians and Guides are the ones who keep everything pieced together, so of course one of their curse words is "Guides and Guardians!" Of the shattered pieces, there are light landscapes and dark landscapes, and winding up in a dark landscape can be pretty terrible, so another curse they have is "Daylight!"

All pretty innocent to us, but we see the logic in it as readers, because we do the same thing. Americans are most likely to swear using some figure from Christianity ("God!" "Jesus Christ!") or a place ("Hell!") or religious action ("D*mn!") or by sex (F-bomb ahoy!). Two things that obsess us are religion and sex. This pattern is repeated in history, where more devout folks than us got a lot more creative ("By the devil's hairy arse!"), and I've also seen it reflected in other parts of the world.

If you want curse words but want to keep it light, decide what's important to your characters. Do they superstitiously worship any form of sea god? They could curse by that ("By Poseidon's trident/hairy nostrils/barnacled bum!"). They could also curse by what's important to them or by what they fear most ("Sails and stormy waters!"). It really depends on what's appropriate in your world.

Also, for some truly nasty insults that use few, if any, current curse words, check out insults thrown in Shakespeare's plays. I mean, seriously, the man could craft an insult:

  • “Away, you starvelling, you elf-skin, you dried neat’s-tongue, bull’s-pizzle, you stock-fish!”
  • “Peace, ye fat guts!”
  • “Poisonous bunch-backed toad!"
  • “That trunk of humours, that bolting-hutch of beastliness, that swollen parcel of dropsies, that huge bombard of sack, that stuffed cloak-bag of guts, that roasted Manningtree ox with pudding in his belly, that reverend vice, that grey Iniquity, that father ruffian, that vanity in years?”
  • “Thine face is not worth sunburning.”
  • “Thou cream faced loon”
  • “Thou elvish-mark’d, abortive, rooting hog!”
  • “Thou leathern-jerkin, crystal-button, knot-pated, agatering, puke-stocking, caddis-garter, smooth-tongue, Spanish pouch!”
  • “Thou lump of foul deformity”
  • “You poor, base, rascally, cheating lack-linen mate!"
  • “You starvelling, you eel-skin, you dried neat’s-tongue, you bull’s-pizzle, you stock-fish–O for breath to utter what is like thee!-you tailor’s-yard, you sheath, you bow-case, you vile standing tuck!”
  • “Your brain is as dry as the remainder biscuit after voyage.”
  • “Virginity breeds mites, much like a cheese.”
  • “Villain, I have done thy mother”

If you're looking into things that can be said during altercations, you could do worse than making up a few insults like these. He took what was considered disgusting, offensive, or beneath contempt and had a field day using those comparisons to insult people. It would work, and it might even go well the stereotypical pirate lingo.