Río Sidra [1] is about an hour from the Porvenir/Cartí area by motorized boat. Río Sidra has a well-established airstrip, but neither domestic airline is currently listing flights there.
The El Llano–Cartí Road appears to be to blame, as most tourist traffic consists of backpackers heading to the Islas Robinson, and most of these are taking the land route to Cartí [2] and then going by boat to their island of choice.
The airlines frequently add and drop routes, however, and it’s always worth giving them a call because planes will sometimes make unscheduled stops if there is enough demand.
Note that the Río Sidra airstrip is actually on the mainland coast rather than the island itself.
Links:
[1] http://www.moon.com/destinations/panama/kuna-yala/the-rio-sidra-group/rio-sidra
[2] http://www.moon.com/destinations/panama/kuna-yala/the-el-porvenir-group/carti