Charlton Heston vs Zombies-
You might be thinking of the Omega Man? Not quite zombies (more like vampires), but kind of close.
Power Armor-
I would think being in power armor essentially makes you spam in a can. If enough zombies overpower you, then they will just get your armor off and eat you.
Well, problem with zombie sharks is that one theory of zombies is that the zombie virus or zombie infection essentially repels carnivores.
So its possible that the shark will get a whiff of the zombie and think, "Fuck that, I ain't eating that thing."
At the same time, the zombie might bite the shark (unless the zombie is human meat only).
As to the island defense-
(1) don't assume that the island is safe. Remember, currents move past islands too and currents could carry your zombie from one location to the other.
Ok, perhaps the ocean currents might create a giant Pacific Zombie Patch, like the Great Pacific Garbage Patch-
http://en.wikipedia.org/wiki/Great_Pacific_Garbage_Patch
You might not get lots of zombies coming ashore (assuming zombies float) but the occassional zombie might surprise you.
Lesson of zombie fighting- the zombies come when you least expect them.
So either zombies sink or zombies swim, either way, they can come to get you. All I am saying is don't think the ocean is a barrier. It might not be.
Building a wall is a good idea.
That said, a zombie that has been in the water for a month is probably in damn ugly shape.
(2) The second problem is, as pointed above, raiders or other people.
Raiders or adversaries-
http://www.zombiesurvivalwiki.com/page/Raiders
There are a lot of uninhabited islands out there, but most are pretty close to populated islands. Populated islands can easily suffer zombie infestation too.
Ok, but even if you manage to find an isolated uninhabited island, you still have the problem of everyone else wanting to live there too. Face it, chances are lots of folks will have the same idea.
Next thing you know, you'll have lots of folks fighting over limited resources or at least, bumping elbows in a very populated island.
The challenge is one of resources. Ok, perhaps you can fish for food, but what about everything else?
There are a lot of potential refuge places, but almost all have some kind of negative side to them. Go north, you can freeze zombies, until summer thaw. Then you get defrosted zombies.
Islands, you get washed up zombies.
Fortress is fine, but then you got to have a fortress where you can also produce food- a siege lifestyle- kind of sucks.
