I don't understand exactly what you mean by "islands".?? There are two different concepts that can be called an island.
The extended family chart (usually) creates a tree / island for the extended family of a SPOUSE of a person. The two "trees" or "islands" are then connected by the number that spouse is assigned. The first spouse in the first tree may be given the number 1. When you see a spouse with a number inside their box, you need to find their family tree by looking for that number in another tree. You are given a finding aid by FTM by a box in the lower right corner, which presents a pick list with cross references of instances of a person in these different trees (which can look in the report as if they are island because they stand alone).
This concept is completely different than a real island in your database as compared to some base person. In that case, that real island in the family tree chart also shows up as a separate tree, but no number leding into it from another tree; but spouses in that tree may "fork" off into other trees.
Using the Extended family chart to identify real islands in your database is very difficult, especially if you have a large tree.
If you have the box checked for including everyone in your database, it will do just that. If you uncheck that box, teh EFC should show everyone related to the "selected person" by blood or marriage. With the unchecked box, you will not see real "islands", they are separate trees that are trees of spouses or spouses of spouses, etc.
If you right click - delete everyone in your EFC, that should leave unrelated people, or islands, to the selected person, alone in the file. (Do this only with a copy of your file - suggest a unique name, of course.)
This is somewhat hard to explain in words. I suggest you create a "test" or "play" tree (I have a play tree that I use all the time) and create one family, then another connected one, then a completely unconnected one and study the Extended Family Chart with that simple example. It's easier to see it visually than it is to explain it in words.
The best place to find multiple connections between people is at:
People Workspace > Main Menu > Tools > Relationship calculator - look under the down arrow to see multiple relationships.