The navigation computer needs the NavComp part to work
The computer will take care of the rest, if it's still working. It'll sail right out to the oil rig, and they won't fire at you because you've got the IFF transponder. Just make sure you've got what you need to access everything: FOB, working computer, and fuel. Easy enough, huh?”— A. Ron Meyers
In order to sail across the ocean to the Oil Rig, the Poseidon Oil tanker needs fuel and a working Navigational Computer. This is an assignment composed of two parts, first you need to retrieve the Tanker FOB from Navarro and the NavCom parts from Vault 13. Then comes the hard part - you need to navigate through the monster-filled tanker hold in order to reach the access point to the navigational computer. Once there, use the FOB to gain entry, go up and install the parts.
Take note, the passageway to the computer is narrow, so its best to temporarily leave the companions downstairs.