island base, eh?
1) you may want to raise the terrain a bit so that in storms it is not knee deep in water in your home. Its harmless -- your fires go out, but they are right back up and running when the waves roll out, but I personally found it annoying.
2) you want to be able to get onto your island from the water if you end up inadvertently or intentionally even going for a swim.
3) you probably want a boat dock. combine 2 and 3 and shape the terrain and such accordingly.
4) its worth the time to find a decent island. There are a great many that are way too small, and you realize it about half way through your build. If you are not great at estimation, try putting down 2x2 floors the size of your new mansion and make sure everything fits.
5) if it does not fit, you can raise terrain a goodly way out into the water, but eventually you will hit a max size. This is a lot of trouble, if you can find a bigger island to start with instead.
6) you want to be far enough from shore that enemy on the shore can't really see and notice you. Its not that far to do this; my best island has a land bridge of sorts that you can wade to the mainland and enemy can wade or swim to me, but its far enough that they don't because they can't see me or notice me at that range. If you can't hit them with a bow from your home, they usually won't see you. (assuming that you can't hit them because of range, and not because you can't shoot well).
7) do not build on islands with barnacles all over it :P

Good luck finding a spot!