Here´s what Encyclopaedia Britannica has to say on the subject:
There are about 15 main Swahili dialects, as well as several pidgin forms in use. The three most important dialects are kiUnguja (or Kiunguja), spoken on Zanzibar and in the mainland areas of Tanzania; kiMvita (or Kimvita), spoken in Mombasa and other areas of Kenya; and kiAmu (or Kiamu), spoken on the island of Lamu and adjoining parts of the coast. Standard Swahili is based on the kiUnguja dialect.
And speaking of Arabic, Swahili has received a great number of Arabic loanwords--including "Swahili" itself!