Originally from the The SMAC Academy, by Colony Pod and Northswordsman.
Once, while playing a watery map, I found that it was nearly impossible to watch for incoming invasion forces. Mighty sensor nets that wrapped my island weren't quite enough, since I could only see 2 tiles away. That will definitely not warn anyone. Then, I came upon an idea. I went into the Unit Workshop and took a foil and a cheap module, the transport*. To defend against IOD's (isles of the deep) cruising along, I put on trance and resonance armor (always use on nonmilitary units). Finally, radar. I had an industrious city pump out about 10 of these Offshore Radar Platforms and I strung them along my island at the edge of shoreline sensor arrays. With a few needle jets in coastal or water bases, no one's getting through a good ORP network.
(* Alternatively, you can use a Trawler setup, as you can harvest the square you are sitting on while watching out for unfriendly units, adding efficiency and paying for itself. Add deep radar and res-armor, as well as trance, for protection. It probably won't insure survival, but it will cause some damage to your opponent, and that is what you want. It's also a "clean" (no support) unit, while you have to pay for the transport unless you add Clean Reactors Ability to the transport, which means you lose trance or radar.)
Another idea is to combine this with Vel's Doctrine: Defense solution, establishing Sea bases around your main landmass to act as coastal defense stations, then using the ORP/Radar Trawlers as expendable pickets to fill in the gaps between islands near your landmass and at choke points around the map for early warning. Backing up the ORP/RT's with a Sea base (Efficient) or a Carrier (Not so Efficient) with Air units allows you to demolish anything that comes through those pickets. This adds an extra layer to your multi-layered defense. Any enemy that comes near you will have to go through some tough defenses just to reach your landmass, where you will be waiting for him armed for bear. Any attacker simply will not have a chance.
If you are unable to set up a Sea base network fast enough, ORP/RT can fill that role until you can. Just crank them out at your best industrial seaside city as fast as you can, rush-building as needed.