This maybe obvious to some but I only just noticed it.

My Danish trading fleet stretched from the Baltic to the eastern Med but because the Italians and Scillians are so territorial about their waters I had to maintain large fleets there and being so far from home this cost a lot to support each turn.

A simple way to reduce these costs is to occupy a couple of coastal territories en route. For example, I have occupied Wessex (gives me cheap sailing as far as southern Spain) and I'm now looking for a suitable candidate in the Med, Malta or Sardinia for example. Build ports in these territories and the support costs of your ships will be reduced dramaticly. Better still if the 'pitstop' provinces produce a few trade goods too.