Walk swim or fast travel. If your not a full Vampire yet and just have the disease Sanguinare Vampiris a potion of cure disease or a shrine will cure it but if your fully turned go to Falion.
What island? You mean Solstheim? You just need to get back to Skyrim and see Falion in Morthal.
Leave, feed on someone and then go back to him.
If you have the Legendary edition you should have Dawnguard and that should have stopped npc's attacking full un-feed Vampires.
Without Dawnguard if a Vampire player doesn't feed after four days you will get attacked. Reload a save if you can before he attacked you (or escape and feed), find a sleeping npc sneak up activate them and select feed then npc's won't attack you for another four days.
You have to hear the rumors from inn keepers first before getting Rising at Dawn.
No they all have the dialog option but you might have to ask a few times (or you might just get it straight away).