In fact, both evaporation and crystallization cannot form pure salt.
Salt form by crystallization is just more pure( NOT pure!! ) than that by evaporation.
Only neutralisation with exactly the same amount (EXACTLY) of acid and alkali can form pure salt.
First, heat the sea water until the solution is saturated.
Then. keep it for a few days for the water to evaporate slowly (don't cover it!!).
You will then find some pieces of crystals(NaCl) which is quite pure.