Hello, On my very first Galactic War game, the opposing commander had his base in some kind of ocean. I found it a bit lucky to be able to destroy his commander with my fabricators building torpedo launchers at the edge of the ocean. Or perhaps, it's willingly done this way, but I really felt : confused : how can I kill him, I have no air units, no sea units, nothing from the orbit that can launch stuff? depressed : I don't even have any artillery at my disposal... exasperated : my fabricators are not amphibious in this game!
I feel like most of the basic loadouts have the tools capable to theoretically win in any theatre. Torp launchers are actually pretty awesome against naval, and you've always got unbrellas to deal with orbital etc. (the exception to this is the BACON commander, who cannot build umbrellas though maybe I'm mis-remembering.)
You could have made unit tanks if they could have reached this commander, and you could have even sent in your own commander to the fight and use walls and turrets as your commander is amphibious. This has only happened to me once however, but I had aircraft at the time.
Torpedo creep is the only viable strategy I've found in this situation, given that you have nothing land-based that can reach. Incidentally in my last game doing this, the AI commander suddenly decided he was feeling too wet and came back up on land (i.e. my base).