Is it possible to move resources around?

Yes it is possible, in the order to deliver the items select Transfer and select Do not load. This will also prevent the vehicles from registering a negative value.

Source: http://wiki.openttd.org/Orders#Transfer

Transferring is the best method, but you can also use station walking to make the dock accessible.

Create a line of stations from the coal to the water, and then build a dock. The entire segment will be considered a single station, and so has access to the coal even though it far away. You can delete the middle stations and still have them be connected.

This can be seen as cheating, especially if the distance is extremely far.

I did this by letting the boat load (fully), and then setting the target (intermediate) station to Unload.
The boat will correctly load and unload as desired and you can then transport the coal in question however you like.

As a side note, if you are rich and don't care about the environment, you may simply rip a new channel into the target island. ;)