I suggest making a new suggestions topic. Always best to get opinions of your idea in your own topic, rather than bounce it in another persons.
As for the original suggestion, it’s not a bad idea really. However, what we must always think about is abuses. Because everyone loves to abuse things. As most already said, this eliminates choke points to a point. Especially if you can create an unlimited amount (even if more costly). Part of what makes Earth so special is it’s choke points throughout the map which can really help limit people from accessing areas. With this, it would destroy a lot of people’s strategies.
What may work however is putting both a limit of 1 per player AND a very high resource cost to make (perhaps something like 2500 metal, 1000 oil, or if we want really steep, 5000 metal, 2500 oil). And as you stated, making it sinkable via nukes would add a strategic value to it. Especially if you actually make it so it’s truly ‘sinkable’ and any units on it when hit by nuke would additionally kill all units there too. This would prevent people from stacking units in the sea as the risk could easily be catastrophic. This would help with some issues of camp walling (which can truly be a pain to play against), but not completely break it as you have a way to thwart the counter as well (nuking the water OP). So it could be just as dangerous for an enemy to use the OP to enter your territory and not have an easy retreat incase they mess up.