divers are basically invisible at night.

In James bond movies yes. But most are written well before drones and very cheap thermal cameras.

It is pretty easy with modern equipment to monitor body heat in water. With the addition of AI very easy to ID human Vs fish both by shape and differing body temps.

But I'm no more a real general then yourself. So back to the armchair.

Because if it was that easy, they would have done it?

There are quite a few smart people versed in tactical warfare racking their brains on how to break Russian supply lines. If they haven't come up with a feasible plan to blow up the bridge yet, I doubt you just did.

My (armchair general) assessment:

• Way behind enemy lines in one of the most guarded areas of occupied Ukraine territory.
• No covert means to deploy or retrieve divers. The Ukrainian navy doesn't operate submarines.