Use something with at least 40 percent DEET. You've got to reapply, esp. to your feet and ankles, when walking the beach because you get your feet and ankles wet, mon!
After you get bitten, try ammonia to desiccate the bite(or AfterBite, which is ammonia), then a steroid cream(hydrocortisone works OK, but if you have something stronger...mometasone furoate, for example, use it instead) to relieve the itching and swelling...DON'T SCRATCH. You don't want to get an infection on vacation, do you???