Presence of Transporter
You may only build on tiles in which either a land transporter, or boat in a river, or a docked boat of your own colour is present
Only oil rigs may be built on a sea tile
Does this mean to build an oil rig you have to put the required material onto a land transporter, and then load that transporter onto a boat and then sail out to sea to build the oil rig?
nope. that would be forbidden move. transported vehicle can't carry materials or another transporter.