VoIP = Skype.

No, VoIP is based on open, published standards. Skype is a proprietary hack
with nothing published regarding it's format or protocols.

It only costs you a little relay
bandwidth you'll never notice as Skype routes other users data

You say this based on what experience and measurement from an actual marine
environment? p2p traffic is not without it's waste of bandwidth. For
someone in a limited bandwidth situation it may well be a BIG problem to go
"sharing" it for p2p apps like skype.