A pair of Portals need 2 things:
- A matching name (the default empty "" also works)
- No portal disrupting items in your inventory (metal ore and metal bars)

It's a good idea to use map icons to keep track of portals.

It's a good idea to keep a spare blank portal at home base, and immediately deploy its pair after sailing to shore. It'd be a real shame to die after landing and have no return path.