It depends on whether you want something kind of rustic, or a "real" home away from home.
I didn't notice one very important thing (IMHO) - and that's a woodstove. The power is not 100% reliable, and the stove is VERY nice to have on a cold morning (or evening). I would also include some type of other heat - electric baseboard (particularly in the bathroom)...
Storage space is always at a premium. Put the house up high enough so you can put stuff underneath. Also makes it a whole lot easier when you have to fix something under there!
When you do the plumbing, MAKE SURE EVERY FEED PIPE is on an angle, all pointing "down" with no low spots or water traps anywhere. Put in 2 master drain valves (or more if you need them, but if all is well you won't), 1 on each feed (hot and cold water). The idea is to get all of the water out of the system quickly and RELIABLY when you close up.
More than 1 bath is really nice if you have the space. Preferably on different floors or better still, one if them in an external building
Don't forget closets!!!!
Use good quality windows. It costs more up front, but good windows won't fail or get moisture laden between the panes of glass...
Screened in porch? Depends on the Island and the direction. We don't have one and don't have bugs, but facing West is good in that way.
Someone mentioned a crank up dock. This is a REALLY good idea!