While the briny sweetness of the crabs themselves will flavor your meal, I always add a spice mix to the boil. While you could add anything you'd like, I prefer one of two seasoning mixes: Old Bay crab boil, or Zatarain's crab boil. Old Bay is the classic for Maryland or Virginia crab, while Zatarain's is a Cajun seasoning.
And if you are on the ocean, there is nothing wrong with boiling your crabs in seawater -- so long as it's clean.
At any rate, once your water is boiling away, drop your crabs in one at a time. Don't overcrowd the pot. Boil in batches if you need to.